(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Optical processes in cadmium sulfide thin films"

OPTICAL PROCESSES IN CADMIUM SULFIDE 
THIN FILMS 



By 

EDWAED M. CLAUSEN, JR. 



A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL 
OF THE UNIVERSITY OF FLORIDA IN 
PARTIAL FULFILLMENT OF THE REQUIREMENTS 
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY 



UNIVERSITY OF FLORIDA 



1987 



Copyright 1987 
by 

Edward M. Clausen, 



ACKNOWLEDGMENTS 



Not enough can be said for the people who have contributed to the 
start and particulary the completion of this dissertation. It would 
have been very difficult for me to get through this project without 
their help. 

I would like to express my deepest appreciation and gratitude to 
Dr. Joseph H. Simmons, my academic adviser, who supplied much 
encouragement, direction, assistance and the opportunity to work on 
this project. His insight and ability to help me solve problems proved 
to be an extremely valuable asset, although his confidence in my 
abilities to "get the job done" was invaluable. I would also like to 
express my appreciation to the members of my committee. Dr. Paul 
Holloway, Dr. Robert Dehoff , Dr. Stan Bates, Dr. Tim Anderson and Dr. 
Ramakant Srivastava for their suggestions, comments and guidance. This 
is certainly one of the best combinations of abilities and expertise 
for a committee and I thank the members for their help and enthusiasm. 

My closest friends Guy Latorre, Richard Robinson and B.G. Potter 
deserve special thanks for their personal support and reassurance. My 
roommate and friend, Steve Wallace deserves the most thanks and credit 
for having to endure the more trying times of this project. I would 
especially like to thank my mother and father for their support 
throughout all of my endeavors, and my closest and dearest friend Laura 
Harmsen for her inspiration and emotional support. 

iii 



Finally, I would like to express my graditude to the Air Force 
Office of Scientific Research (AFOSR 84-0395) for funding this research 
project. 



iv 



TABLE OF CONTENTS 



PAGE 



ACKNOWLEDGMENTS iii 

ABSTRACT vii 

CHAPTERS 

I INTRODUCTION 1 

Optical Signal Processing 1 

Nonlinear Optical Materials 2 

Thin Films 5 

II THEORY OF NONLINEAR OPTICS 8 

Nonlinear Optical Susceptibility 8 

Optical Bistability 19 

III BACKGROUND - BIBLIOGRAPHICAL REVIEW 28 

Bulk Properties of Cadmium Sulfide 28 

Photoluminescence of CdS 32 

Resonant Raman Scattering in CdS 37 

Nonlinear Susceptibility 38 

CdS Thin Films 39 

Vacuum Deposition 39 

RF Sputtering 45 

Semiconductor Doped Filter Glasses 50 

IV EXPERIMENTAL METHOD 52 

Vacuum Deposition System 52 

RF Magnetron Sputtering 60 

Thin Film Deposition 63 

Substrate materials 64 

Substrate temperatures 66 

Deposition rates 68 

Co-Sputter Alternating Deposition (COSAD) ... 68 

Post Deposition Treatments 70 

Thin Film Characterization 71 

Microstructure 71 

Chemical Analysis 73 



V 



Optical Measurements 76 

Index of Refraction 77 

UV-VIS Absorption 86 

Photoliuninescence and Raman 88 

V RESULTS AND DISCUSSION 96 

Thin Film Physical Properties 96 

Microstructure Characterization 96 

Transmission electron microscopy 96 

Scanning electron microscopy 109 

X-ray diffraction experiments 123 

Compositional Analysis 132 

Stoichiometry determination of target material 132 

Analytical techniques 134 

Co-Sputter Alternating Deposition 149 

Microstructure 149 

Composition 163 

Thin Film Optical Properties 165 

Absorption Spectra 167 

Room temperature UV-VIS absorption 168 

Low temperature UV-VIS absorption 174 

Index of refraction 185 

Photoluminescence and Raman Spectroscopy .... 190 

Photoluminescence spectra 193 

Raman spectroscopy 207 

COSAD Optical Properties 211 

VI CONCLUSIONS 218 

VII FUTURE WORK 224 

APPENDICES 

A BASIC PROGRAM FOR LAMBDA 9 SPECTROPHOTOMETER ... 227 

B ELLIPSOMETRY 230 

BIBLIOGRAPHY 235 

BIOGRAPHICAL SKETCH 240 



vi 



Abstract of Dissertation Presented to the Graduate School 
of the University of Florida in Partial Fulfillment of the 
Requirements for the Degree of Doctor of Philosophy 

OPTICAL PROCESSES IN CADMIUM SULFIDE 
THIN FILMS 

by 

Edward M. Clausen, Jr. 
December, 1987 

Chairman: Joseph H. Simmons 

Major Department: Materials Science and Engineering 

The semiconductor, cadmium sulfide, has received much attention 
for its optical, electronic and piezoelectric properties. Recently, 
several investigators have demonstrated that its Wannier excitons hold 
great promise for applications in nonlinear optics. However, the 
results showed, as expected by the current theoretical thinking, that 
the nonlinear optical behavior and the exciton and band gap energy 
configurations are dependent upon the microstructure of the samples. 
Since most applications in computer logic, or commtinications require 
waveguide geometries, the object of this dissertation has been the 
study of thin films. 

The investigation presented here, therefore concerns the study of 
the effect of microstructure and preparation conditions on the optical 
properties of CdS thin films. High optical quality films were produced 
by RF magnetron sputtering with a variety of microstructures and 
crystallographic characteristics controlled by the deposition process 

vii 



and subsequent heat treatments. An in-depth study of the thin film 
microstructure revealed the relationship between crystallographic 
defects and band gap defects which lead to band tailing absorption. 
Optical characterization related exciton photoluminescence, absorption, 
resonant Raman scattering and band edge photoluminescence and 
absorption to the microstructure of films. For the first time, 
absorption bands and photolximinescence associated with exciton states 
were observed in a polycrystalline thin film. 

A supplementary study investigated composite films, consisting of 
CdS crystals in a glass matrix, formed by a novel co-sputter deposition 
process, in which alternating layers of CdS and a borosilicate glass 
were deposited to form a thin film. A wide variation in the structure 
of the deposited film was obtained by changing the amount of deposited 
CdS and by post deposition heat treatments. Low concentrations produced 
CdS microcrystallites as small as 70 A, small enough for quantum 
confinement processes to affect the band energy. For all films 
produced, a shift in the band absorption edge to higher energies was 
observed; however, it was determined that this shift must be partially 
associated with a chemical shift. Possible formation of CdO could 
raise the band gap energy, although this alone could not produce the 
shifts observed in the films with the smallest particles, and therefore 
a confinement process might exist. 



viii 



CHAPTER I 
INTRODUCTION 

Optical Signal Processing 
With the very intense development of optical commiinications in 
recent years, the need for integrated optical systems for the 
processing of optical signals has greatly increased. To take full 
advantage of the speed and information bandwidth available at optical 
frequencies, an all optical system is desired. All of today's systems 
operate by the high bandwidth transmission of optical signals in 
optical fibers, followed by conversion to lower bandwidth electrical 
signals before any processing, such as amplification and multiplexing, 
can take place. The limiting drift velocity of charge carriers in a 
semiconductor and the capacitive coupling between adjacent elements 
present the fundamental limit for processing speed in these systems. 
Also, the serial nature by which the electronic data must be 
manipulated presents another speed barrier. An alternate approach 
would utilize optical bistability and optical switching demonstrated in 
certain materials for all-optical modulation, detection and 
multiplexing. Optical switching is achievable with materials which 
display a third order nonlinear susceptibility. A third order 
susceptibility leads to an intensity dependent index of refraction or 
absorption. This nonlinear refractive index can exhibit onset and 
decay on a very fast time scale, which makes it an attractive 



2 

phenomenon for optical signal processing. With the proper material, 
all optical operations could conceivably be carried out in a monolithic 
thin film, which would act as a guiding medixom with both active and 
passive regions. An optical communications fiber could be coupled 
directly to this thin film, thereby fully utilizing the speed and 
bandwidth of the optical signal. 

A complementary application of an integrated optics technology 
which utilizes optical switching would be in the area of high speed 
logic operations for the next generation of computers. Logic gate 
operation has been demonstrated with several materials which exhibit a 
nonlinear optical susceptibility and optical bistability. A few of 
these gates have been shown to switch on a subnanosecond time frame, 
which is competitive with present day high speed electronic systems. 
The primary advantage of the optical gate is the possibility of 
parallel processing on the fundamental logic cell level, which adds 
tremendous speed advantages over electronic systems. Most optical 
computer designs today are based on integrated optics, in which the 
active regions consist of arrays of bistable devices arranged with a 
high spatial density. A high density of gates is possible because 
optical gates are not subject to capacitive coupling, thus making 
possible massive parallel processing without the connection problems 
encountered in today's electronic systems. 

Nonlinear Optical Materials 
A large number of architectures have been proposed for both 
computer systems and multiplexing circuits based on optical switching. 



While this field has greatly advanced, there is, however, a very great 
need for suitable materials and systems. Many materials exhibit a 
third order nonlinear susceptibility,^ but very few have the 
characteristics necessary for an integrated optics system. A few of 
the semiconductors which have been investigated include InSb, GaAs, 
GaP, CdS, and CdTe. Most of these semiconductor materials which show 
optical switching have been investigated in bulk form. Only the 
multiple quantum well (MQW) structures made from gallium arsenide are 
the notable exception and are made in thin film form.-^ This material 
is perhaps the most promising today for use in optical signal 
processing systems, primarily because its nonlinearity occurs at the 
same wavelength as the semiconductor lasers which are currently 
available. For an integrated optical system this is an essential 
consideration because most of the processes which produce the nonlinear 
susceptibility require laser light of energy near the band gap of the 
material. 

Equally important considerations, however, include the value of 
the nonlinear coefficient, the switching speed, and the absorption 
coefficient, since these values determine how much power is required to 
switch the device, and how fast recovery will be. Low power operation 
is essential for any large scale integration, although the figure of 
merit (FOM) most often quoted is given by 

FOM = "2 (1.1) 

X a 

where n2 is the nonlinear index, x is the switch-off time, and a is the 
absorption coefficient. Obviously the larger the FOM the more 



1 



4 

attractive a system is. MQW gallixam arsenide has an additional 
advantage of room temperature operation, but suffers from a slow 
switch-off time and very large absorption. There are other materials 
which have been shown to exhibit a much larger nonlinear effect than 
MQW gallium arsenide. A material which has been demonstrated to exhibit 
one of the largest nonlinear coefficients is cadmium sulfide (CdS). 
The large coefficient was obtained by saturating a bound exciton level 
in the band gap.'^ The mechanism which leads to the large exciton 
saturation effect in CdS is central to this dissertation and will be 
described in a subsequent section. However, only a very few groups 
have looked at CdS, and no one has either investigated the bound 
exciton saturation mechanism in thin films or explored the possible 
applications in integrated optics. 

The most important consideration for any practical nonlinear 
application is the temperature at which the nonlinear process 
predominates. To the present day the largest nonlinear coefficients are 
only measured at very low temperatures. For example, the large 
coefficient obtained by saturating the bound exciton level in CdS 
occurs at 2° K. Since the binding energy of the bound exciton only 
corresponds to a few millielectron volts, the state is thermally 
annihilated at higher temperatures, and the large nonlinear effect 
disappears . 

The electronic structure of the band gap, however, may be altered 
to permit access of exciton levels at higher temperatures by 
controlling the physical size of the material. The phenomenon by which 
this occurs is known as quantiim confinement. When the crystal size of 



i 



a material is on the order of the radius of the exciton state, new 
boundary conditions can distort the translational motion of the 
exciton, its binding energy and the individual orbits of the electron 
and hole. At this size the electron and hole interactions with the 
crystal surface begin to govern the electronic properties of the 
semiconductor. Quantum confinement also occurs in the MQW structures 
of GaAs; however the exciton is only confined in one direction. A much 
stronger effect occurs when the confinement is in two or three 
dimensions. 



Thin Films 

A study of CdS thin films was chosen for a number of reasons: 1) 
very few semiconductors which display a nonlinear susceptibility have 
been tested in a thin film form, despite the potentially dominating 
role in applications. 2) Cadmium sulfide displays one of the largest 
excitonic saturation effects, and thus the influence of film structure 
due to deposition conditions or subsequent treatments could be 
investigated. 3) Quantum confinement effects and their interaction with 
the perturbation of the exciton absorption process are of great 
interest to the future of nonlinear optical developments and 
applications. Cadmium sulfide promises to offer a means of studying 
both the effects and their interplay with the development of the thin 
film structure. 4) Since the energies of the exciton states correspond 
to wavelengths in the visible part of the spectrvim, the experimental 
optics for the measurement of these states is simplified, and many 



6 

investigative tools become available for following the underlying 
processes . 

The study of any thin film for this application should start with 
examining the properties of the bulk material which contribute to the 
specific origin for the nonlinear effect. In the case of CdS this 
means looking at the electronic states of the material which lead to 
the presence of excitons. These states have been thoroughly examined 
for more than 30 years and they are probably the best xinderstood in 
this material. Exciton states have been shown to occur in thin 
epitaxial films, but no one has investigated the presence of these 
states in polycrystalline thin films, nor have the effects of 
preparation conditions on the excitonic transitions of a thin films 
been investigated. The objective of this study therefore is to 
determine how exciton states would occur in polycrystalline thin films 
of the material, how they are affected by structure and formation 
conditions, and how size variations and grain boundary structures might 
affect their energy level structure. 

A supplementary part of this study will investigate the 
possibility of producing thin film structures consisting of a glass 
matrix with small isolated crystals whose sizes matched those needed to 
develop quantum confinement effects. The investigation will use 
spectroscopic techniques to determine if quantum confinement effects 
can be induced on the exciton states in the material. The objective is 
to produce a thin film of semiconductor doped filter glass. 

The process of quantiam confinement has been and still is under 
investigation in bulk semiconductor doped filter glasses, which have 



7 

received considerable attention lately for nonlinear optics 
application. These are crown base glasses which contain one or two 
percent of CdS or mixtures of CdS and CdSe. It has been postulated 
that the semiconductor crystals exist in the glass matrix as finely 
dispersed microcrystallites, which are small enough to permit quantum 
confinement effects to occur. This effect is still not well understood 
and has not been clearly demonstrated. Many authors have observed 
energy shifts that may be due to compositional effects rather than 
microstructure size. However, it appears theoretically that with 
sufficient confinement, the exciton state will be accessible at room 
temperature. 



CHAPTER II 
THEORY OF NONLINEAR OPTICS 



Nonlinear Optical Susceptibility 
The nonlinear refractive index which is observed in certain 
semiconductor materials is a result of an electronic polarization which 
is induced by the interaction with a monochromatic radiation field. 
The susceptibilities of the polarization determine the values of the 
experimentally measured optical properties. To understand the 
relationships between the susceptibilities and the optical properties 
we must first consider the electro-magnetic field, E, which is given by 

E(t) = E(a))e-i'^t + E*(a))ei'^t _ (2.1) 

The resulting polarization has frequency components at all multiples of 
+/-U), but considering only those that occur at co 

P(aj) = X^^^E(aj) + x^^^[E]E(w) + x^^^ [E]2e(u)) + . . . (2.2) 

The first term in the series, x^^) is the linear susceptibility and by 
using first order perturbation theory,^ the linear dispersion of the 
refractive index below the band gap can be calculated. The higher 

8 



9 

order terms in the series are the nonlinear susceptibilities, and 
although their magnitudes are much smaller than the first term, under 
very high field intensities a number of different effects can be 
observed. For non-centrosymmetric crystals (i.e. crystals without an 
inversion center), under appropriate conditions, the second order term 
is manifested as two different effects. The first is a quadratic 
variation of the refractive index with applied voltage, which is known 
as the Kerr effect. The second is the generation of a second harmonic 
radiation field. Second harmonic generation is a very useful effect 
for doubling the frequency of a laser beam. Both of these effects have 
many important applications; however, the primary interest of this 
study is the effects which lead to the third order term x^"^^- One 
consequence of the third order term is the generation of a third 
harmonic radiation field. For the current topic of this study, 
however, the manifestation of this term as a nonlinear refractive index 
is of primary interest. The relationship between the third order 
susceptibility and the nonlinear refractive index can be understood by 
first considering basic dielectric theory for the displacement of a 
charge in response to an applied electric field: 

D(a)) = E(aj) + AttPCo)) = e E(cj) (2.3) 



where D(a)) is the frequency dependence of the displacement and e is the 
complex dielectric constant. The variable, e can be defined as 



10 

e = (n + ) ^ (2.4) 

where ca/2u) is the extinction coefficient. By combining these two 
equations a relation between the polarization and the index of 
refraction can be written. 



(n + icq 1 + An P(a))/E(u)) . (2.5) 

2w 



The nonlinear susceptibility can be defined by expansion of the 
refractive index in terms of the intensity I, of the radiation inside 
the sample:^ 

n = n^^ + n2l + n2l^ + . . (2.6) 



Next we assvime that the extinction coefficient is very small compared 
to n. Then by using equations 2.5 and 2.6 and expanding the terms. 



n^ = n^ + 2n^n2l + (n^D^ = 1 + 4ti x^^'* + ^ir x^^'*[E]^. (2.7) 



Finally we assume is much smaller than n^^ and by comparing 
coefficients of [E]^ and I 



11 



^ ~ 1 + Att X 



(1) 



(2.8) 



n 



and 



2iT X 



(3) 



n. 



•2 



(2.9) 



n 



1 



There are essentially four electronic processes which can produce 
a reactive nonlinear susceptibility in semiconductors. These are known 
as 1) the induced free-carrier plasma, 2) the dynamic Burstein-Moss 
effect, 3) the direct saturation of interband excitations, and A) the 
saturation of exciton absorption.^ Of these 1 and 2 are the most 
commonly studied, 3 occurs in most direct band gap semiconductors, and 
4 is the most promising for high speed operations. All four processes 
can occur in some materials. The most dominating process which is 
observed depends somewhat on the material, but mostly on the particular 
experimental setup and measurement temperature. As shown in Table 1, 
different processes result in widely different values of the reported 
nonlinear index and saturation intensity. 

The four processes listed above basically describe how the 
transitions between different levels in a semiconductor and the 
saturation of those levels result in the observed nonlinear 
susceptibility. Depending on the band structure of a material and the 
particular wavelength of light used for the analysis, one of the four 
processes will dominate. The one process that can occur in nearly 
every semiconductor, however, is the induced free-carrier plasma. 
Assuming that photo induced transitions produce electron-hole pairs, 
the number of these free carriers will be intensity dependent. 



12 



TABLE 1 



Listing of Nonlinear Optical Values for Certain 
Semiconductor Materials 



Material 


Electronic 
Proces 


Ig (W/cm^) 


n2 (cm^/W) 


toff (sec) 


FOM 


GaAs 








10"8 




(MQWS) 


FES 


150 


10-^ 


1 


Bulk 


DBM 


500 








InSb 


DIS 


2000 


6X10"5 


10^^ 


0.06 


CdS 


BES 


58,26 


io-'^,io"2 


10-9 


2 




FCP 


1.2X10^ 




10-10 





Key for Electronic Processes: FES - Free Exciton Saturation 

DBM - Dynamic Burstein-Moss 

DIS - Direct Interband Saturation 

BES - Bound Exciton Saturation 

FCP - Free Carrier Plasma 



13 

Depending on the recombination time Xj^, and the absorption coefficient 
a, the steady state density of free carriers will be given by 



a I T 

N = . (2.10) 

h w 



Once these carriers are formed, they are allowed to diffuse and form an 
electron-hole plasma. The plasma will respond to an applied electric 
field and the resulting undamped oscillations will produce a 
polarization which can be related to the index of refraction through 
the dielectric constant^ 



2 

n = ( e - ). (2.11) 

* 2 
m Oi 



By use of equation 1.10, the nonlinear refractive index for a plasma 
can be written as 



- 2 -rr e a 

n„(P) = (2.12) 

^ * 3 

n m oj 



14 

where n is the refractive index of the material without the plasma and 
m is the effective mass. The transient susceptibility is determined 
by the time it takes for the free carriers to build up;^ however, this 
can be a very short time. This process will also occur at room 
temperature. The disadvantage of this process for nonlinear optical 
applications is that the effect is very small because there is no 
coupling between the states, and therefore no resonance effects take 
place. The process also can suffer from a very slow recovery time 
because the recombination time in certain semiconductors is on the 
order of psec.^ 

Another process that is based on an intensity dependent free 
carrier concentration is the dynamic Burstien-Moss or blocking effect. 
Again a steady state density of charge carriers is given by equation 
2.10; however, the origin of the absorption is not considered 
explicitly. At low temperatures the carriers are assumed to thermalize 
by a phonon scattering process so that they fill the bottom of the 
conduction band. The top of the valance band becomes empty and the 
shift of the effective band gap to higher energy becomes intensity 
dependent. In association with this shift there must be an intensity 
dependent contribution to the refractive index. ^ This is because the 
filled conduction band effectively blocks absorptive transitions, and 
the blocked transitions no longer contribute to polarization and 
refraction. Also, some type of unspecified coupling takes place 
between excited states, so that the effect is enhanced somewhat. The 
nonlinear refractive index for this process is given by 



15 



n^CBM) = ^ ^ ( ^ ^ )^ ^ (2.13) 

3 n h to h ((jjg - (jj) I 

where N is the free carrier density given by equation 2.10, P is a 
momentvun matrix element, and ojq is the effective band gap. For 
nonlinear optical applications the advantage of this process is that 
the occupied states are closer to resonance so that a larger n2 
results. There also is the possibility that this process can occur at 
energies below the band gap, as saturation of excited carriers can be 
produced not by direct optical absorption, but by scattering from other 
excited states.^ The one disadvantage of the process that is similar 
to the induced plasma process is that the interband relaxations 
required for decay of the state can be very slow. In some materials a 
faster decay process can occur by scattering to intraband transitions, 
which effectively relaxes the system by transferring the population to 
other states.^ 

Another mechanism for the nonlinearity observed in some materials 
is by a direct interband saturation process. This process asstimes that 
the band structure in a direct gap semiconductor can be modeled as a 
set of uncoupled two level systems which are homogeneously broadened by 
a dephasing time T2. Homogeneous broadening means that the individual 
transitions are indistinguishable. The T2-Lorentzian broadening 
results in absorption below the band gap and excitation into the T2- 
broadened "band- tail" is assumed to be responsible for the nonlinear 
refraction.^ At some high level of intensity the two level system 
should become saturated, and associated with this saturation is a 



16 

nonlinear contribution to the refractive index, ^ The nonlinear index 
of refraction for this process is expressed by 



2' 



1.3 h 0) T„ ,.2 

h 2 15 n c 



(2m*) (0)^-0) )"^^^ (2.U) 



where x is the relaxation time, and c is the speed of light. The 
advantage of this process for nonlinear applications is that the 
effective nonlinear refractive index is inversely proportional to the 
band gap energy, so for small band gap materials this process leads to 
a very large effect. As indicated above, the broadening results in an 
effect which occurs below the band gap energy, so absorption losses are 
reduced. The disadvantage of utilizing this process is the same as for 
the other two processes; in some materials there is no fast mechanism 
for decay of the excited state. 

The final process that will lead to an electronic nonlinear 
refractive index in certain semiconductors is the saturation of bound 
exciton levels. These particular defect states are characterized by a 
very narrow transition linewidth, which is comparable to atomic 
resonances. A bound exciton is an associated electron-hole pair that 
is bound to an impurity site. The oscillator strength of the bound 
exciton, which is related to the polarizability, is extremely large in 
comparison to oscillator strengths of molecules . In addition, there 
is a very high density of oscillators, which contributes to a very 
large nonlinear effect. The absorption transition of the bound exciton 



17 

is modeled as a saturable two level system, inhomogeneously broadened 
by a T2 dephasing time. The transition linewidth is inhomogeneously 
broadened because excitons bound at different locations see different 
environments.^ An inhomogeneously broadened system is described as a 
distribution of groups or classes of transitions, and within each class 
the transitions are assumed to be identical (homogeneously broadened). 
The saturation of the inhomogeneously broadened system does not depend 
on the homogeneous lineshape function, but rather on the linewidth of 
these "homogeneous packets".^ This means that the saturation intensity 
is inversely dependent upon the dephasing time T2 as shown by 



I = 2 TT^ n^ h V AV 

s 

(j) A 



where Av = (n T2)"^ (2.16) 

and (\) is the ratio of the radiative lifetime to the spontaneous decay 
time, which is usually taken as equal to one. In semiconductor systems 
the dephasing time is on the order of 0.1 psec,*^ which means that very 
low saturation intensities are required to saturate bound exciton 
transitions. The importance of the saturation intensity will be 
described in a following section; however, a small value indicates that 
the nonlinear refractive index is very large, as the two are inversely 
proportional (see equation 2.25). As shown in Table 1, in CdS a 
saturation intensity as small as 26 W/cm^ has been measured, which 
corresponds to n2 value of 1.3 X lO'^ cm^/W.^O This is the largest n2 



18 

value ever reported. Additional advantages of bound exciton saturation 
are that the state decays by a radiative transition and the lifetime is 
on the order of 500 psec. This would make for a very fast, low power 
switch. Also, since the excitons are bound to defect sites, there are 
no carrier diffusion problems, which in the other three processes tend 
to wash out the effect. The one primary disadvantage of utilizing this 
process is that the strongest exciton resonance occurs at 2° K. As the 
temperature is increased, the transition broadens, thereby requiring a 
larger saturation intensity and hence a smaller n2 is observed. In 
addition, since the exciton binding energy is only a few millielectron 
volts, the state is thermally annihilated at higher temperatures. 

As previously described the process of quantum confinement could 
be used to access exciton levels at higher temperatures if the physical 
size of the material could be made small enough. Multiple quantiim well 
structures produce quantum confinement in one direction because the 
structure is made up of alternating layers in which the layers act as 
infinite potential wells and the layer thickness is smaller than the 
exciton radius. Although the exciton is not confined in the other two 
directions, the effect is strong enough that nonlinearity can be 
observed at room temperature. The effect would be larger if 
confinement was made in the other two directions. 



19 

Optical Bistability 
The primary means for measuring nonlinearity in materials is 
through an internal feedback device known as a Fabry-Perot 
interferometer. The saturation intensity of such a system is the 
intensity at which the gain of the feedback saturates. This is the 
point at which optical bistability occurs, and from the saturation 
intensity and the interferometer parameters the nonlinear index of 
refraction can be determined. 

The Fabry-Perot consists of a cavity formed by two plane parallel, 
highly reflecting mirrors. The transmission of monchromatic light 
through the device is determined by the optical path length of the 
cavity. If the cavity is not tuned to the wavelength of the light, 
then a transmission of 1 % results. When the optical path length is 
exactly equal to an integer ntomber of wavelengths, then a resonance 
effect occurs and the output intensity from the device reaches nearly 
100 % of the input intensity. A diagram of this process is shown in 
Figure 1. The optical path length is determined by the physical length 
d, times the refractive index n of the material within the cavity. The 
condition for resonance therefore is given by 

2 n d = ra X (2.17) 



where m is the integer order number. 



20 



Incident 
Beam = 100 




Reflected 
Beam = 90 



Forward 
Beam = 10 




Reverse 
Beam = 9 



Transmitted 
Beam =^ 




Forward Beam Resulting Wave 

/\A/^./^;'^''X/'^/'^AA 



Reverse Beam 



Transmitted Beam 
= Less Thani 
► 




Transmitted 
Beam = 100 



Figure 1 



Schematic diagram of a Fabry-Perot interferometer showing 
how interference of the forward and reverse beams changes 
the output intensity.^ 



21 



When the cavity does not satisfy the above requirement, then a 
linear relationship will exist between the incident and transmitted 
intensity. If a material with a nonlinear refractive index is placed 
in the cavity, then a positive feedback loop will occur where the 
refractive index and the light intensity become mutually reinforcing. 
As the incident intensity is increased, a change in refractive index 
occurs which brings the device closer to resonance, which further 
increases the intensity inside the cavity, which further changes the 
index, etc. This continues until a saturation intensity is reached, at 
which point a phase shift to resonance occurs within the cavity and the 
transmitted intensity suddenly increases. 

The ratio of the incident intensity to the transmitted intensity 
as a function of the phase shift 6 is given by the Airy function A(0) 




= A(e) 



(2.18) 



I. 



1 



where 



A(e) = 



1 



(2.19) 



1 + F sin^(6/2) 



and F is related to the reflectivities of the mirrors R by 



F = 



A R 



(2.20) 



( 1 - R )^ . 



22 




b) 

Figure 2 Descriptions of the operation of a Fabry-Perot. a) plot of 
the Airy function as a function of the phase shift 6, 
reflectivities r, and the finesse F; b) transmitted image 
from a high finesse Fabry-Perot.^-^ 



23 

Figure 2a displays how the relative transmitted intensity and the 
sharpness of the transition is related to the reflectivities and 
finesse of the cavity. As the reflectivity of the mirrors is 
decreased, the finesse is decreased and the transition at the critical 
phase shift broadens. When the reflectivities and the finesse of the 
cavity are large, then the transitions are sharp. For these 
conditions, the transmitted image from a diffuse source through the 
Fabry-Perot will appear as a series of sharp concentric rings, as shown 
in Figure 2b. 

For a nonlinear Fabry-Perot, however, the Airy function must be 
slightly modified to account for the nonlinear index by 



A(e) 



NL 



(2.21) 



1 + F sin ( X I 



eff 



6) 



where x is a constant describing the nonlinear refraction, ■'•^ and Igff 
is the effective mean intensity within the cavity. The total Fabry- 
Perot fractional transmission can be written as^^ 



T = 



( 1 - R )^ ( 1 - A ) 
( 1 - R ( 1 - A ) )^ 



A(0) 



NL 



(2.22) 



Ik 

where the intensity absorption per pass A is given by 



A = 1 - e"°^ (2.23) 



and a is the absorption coefficient. A second equation can be written 
which is parametric in Igff for the Fabry-Perot transmission: •'•'^ 



ad (1-R)(1-A) ^eff ^ {1.1k) 

A ( 1 - R ( 1 - A ) ) I 

o 



The condition for optical bistability can be determined by 
simultaneously solving equations 2.22 and 2.24. A graphical solution 
of these equations which shows the criterion for optical bistability is 
shown in Figure 3. 

The critical intensity 1^, for the onset of bistability is given by 
the intensity 1^ which gives more than one intersection with the line 
and curve. This is the intensity at which the saturation of the 
feedback occurs. Once this saturation is achieved, it is found that if 
the input intensity is reduced, the output intensity does not drop 
until a finite decrease in the input has occurred. In other words, a 
hysteresis effect is observed. The switching between the two intensity 
levels can be considered as a change in logic state. The nonlinear 
Fabry-Perot interferometer therefore can be used as an optical logic 



25 




Figure 3 Graphical solution to the Airy function showing the 
critical phase shift required for the onset of 
bistability. 



26 

gate. The great interest for logic gate applications is that the 
switching between the two levels can occur on a subnanosecond time 
period in some materials. 

The saturation or critical intensity 1^, is found to be inversely 
proportional to the nonlinear refractive index and is given by 

I = _^ . _^ (2.25) 

where 

3 n„ 

3 = (2.26) 

A a 

and M is a figure of merit value for the cavity, relating the 
reflectivities of the two mirrors and the attenuation of light as 
passes through the cavity. Equation 2.25 indicates that for fixed 
cavity conditions, a small saturation intensity corresponds to a large 
nonlinear refractive index. The total index of refraction is given by 

n^ = n^ + An (2.27) 



where 



An = n2l(,' 



(2.28) 



27 

The value of is what actually determines the phase shift but as 
indicated by equation 1.1, the figure of merit for nonlinear optical 
applications also includes the switching times and the absorption 
coefficient. 



CHAPTER III 
BACKGROUND - BIBLIOGRAPHICAL REVIEW 

Bulk Properties of Cadmium sulfide 

Nearly all of the early work on cadmium sulfide was carried out on 

single crystal platelets which were made by a chemical vapor phase 

growth process. The natural crystal structure of cadmium sulfide is 

hexagonal wurtzite, although single crystals of the cubic zincblende 

structure have been fabricated. Within either of the two crystal 

structures it is possible to have regions which are made up of the 

alternate crystal structure. The transition from a hexagonal to a 

cubic lattice or visa versa can occur through a well known twinning 

1 n 

mechanism, •'•^ in which the twinned region is boxind by stacking faults. 
The twinned regions can be manifested during deformation of the crystal 
or under particular growth conditions, although it is difficult to 
differentiate these two sources when crystals are grown from the vapor 
phase. In either case, the two crystal structures do not have a center 
of symmetry or inversion, which leads to the unique properties of 
noncentrosymmetric crystals such as piezoelectricity, pyroelectricity, 
and third order optical susceptibility. 

Another important physical property of CdS is the stoichiometry of 
the crystal. Very little work has been done on the defect chemistry of 
CdS; however, the work done on other II -VI semiconductors such as ZnS 
and CdTe indicates that the range of nonstoichiometry at room 
temperature is very small, e.g. 0.01 to 0.1 zA^ Early work by Collins 

28 



29 

which involved sulfur atmosphere heat treatments and electron 
bombardments, showed that sulfur vacancies were the predominant native 
defect and that they acted as the recombination center responsible for 
the green edge emission associated with CdS luminescence.^^ Other 
studies which investigated impurity doping effects are described in a 
following section on photoluminescence. 

A detailed knowledge of the band gap structure of CdS has come 
from the extensive study of exciton states. Cadmium sulfide is found 
to be a direct gap semiconductor with a band gap equal to 2.59 eV at 0° 
K. The wurtzite lattice of the material is described by a p-like 
valance band consisting of two gamma-7 states and one gamma-9 state, 
and a s-like conduction band made up of one gamma-9 state. A diagram 
of the band extrema is shown in Figure A. The three states in the 
valance band are also known as the A, B, and C free exciton states. 
These intrinsic exciton states are modeled as Wannier excitons; i.e. 
the electron and hole behave like a hydrogen atom. The orbital 
movements of the electron and hole are determined by their effective 
masses within the band extreme. As shown in Figure 5, the solution for 
the wave function of this model results in a series of discrete 
parabolic bands below Eg which merge into a continuum at higher 
energies. Because of the unique band structure of this material 
there are a large number of possible exciton states. Any of the 
exciton energy levels (i.e. n=l,2,3, etc.) can be associated with the 
three primary states (A,B and C) in the valance band. In addition, any 
one of these free excitons can be associated with an impurity center, 



30 



E(k) 




(0,0,0) 
k 



WURTZITE 



Figure 4 Band gap structure for wurtzite crystals near k=0. 



31 




Figure 5 



Energy diagram for Wannier excitons as a function of 
exciton momentum K, showing "hydrogenic" states which merge 
into a continuum at energies greater than E„.^^ 



32 

forming a bound exciton complex, which will have a lower energy than 
the corresponding free exciton. 

Of all the II-VI semiconductor materials, the exciton states in 
bulk CdS have been studied the most and are perhaps the best 
understood. Reflection, absorption, and Iviminescence studies dating 
back to the mid-fifties have investigated the exciton states in this 
material. Excitation of the states can be accomplished by either 
electron bombardment or by photon absorption. When the emission is due 
to the latter process it is known as photoluminescence and the results 
reported for CdS are detailed below. 

Photoluminescence of CdS 

When CdS is excited by photons of energy greater than the band gap 
the characteristic luminescence which results form the decay of excited 
states is shown to consist of two primary emission bands. The first, 
known as the "green-edge emission" is due to the edge emission of 
various states in the band gap, i.e. shallow donor-acceptor 
recombinations. Studies of the edge emission of CdS were made by 
Kroger as early as 1940. ■'■^ At a slightly higher energy, a band known 
as the "blue-edge emission" occurs and is due to emission from free and 
boxind exciton complexes . ■'•^ A typical low resolution spectrum 
displaying these two bands is shown Figure 6, and a high resolution 
spectrum of a portion of the blue band is shown in Figure 7. The 
intensity of the bands is dependent upon the polarization of the 
incident light with respect to the c-axis of the crystal. 



33 




4700 4800 4900 5000 5100 5200 5300 5400 5500 
WAVELENGTH IN ANGSTROMS 



Figure 6 Low resolution photoluminescence spectrxim of CdS single 
crystal showing "green-edge" emission due to band edge 
recombinations and "blue-edge" emission due to excitons. 
Figure shows emissions are dependent upon the polarization 
of the excitation source. 



34 




Figure 7 High resolution photoluminescence spectriim of CdS single 
crystal at 4.2° K, showing bound exciton peaks for 
excitation perpendicular to the c-axis. Peaks labeled P 
are observed with parallel excitation. •'•^ 



35 

The exciton states were first extensively studied and 
characterized by Thomas and Hopfield.^^ They have shown that within 
the higher energy band a nxunber of sharp liominescence lines occur which 
correspond to transitions of both free excitons (A, B, and C) and 
excitons bound to neutral donors or acceptors. A designation of 1^ was 
given to excitons which are bound to neutral acceptors and I2 was given 
to the excitons bound to neutral donors. These two peaks are labeled 
in Figure 7. The distinctions between the various transitions were 
made by using the Zeeman effect. When a strong magnetic field is 
imposed on the sample, many of the luminescent lines split due to the 
spin moments of the ground and excited states. From the group theory 
of bound complexes, they were able to assign the transitions to the 
different defect states. 

A later study by Henry, Faulkner, and Nassau showed for the first 
time donor-acceptor pair lines in the photoluminescence spectra of 
CdS.^'^ These pair lines are narrowly spaced transitions which were 
observed in the green-edge emission band and correspond to closely 
spaced donor-acceptor pair-recombination bands. Again the confirmation 
of these lines was made by Zeeman experiments. The significance of the 
study is that for the first time direct spectroscopic evidence for the 
existence of these states was made. This is important point because in 
the study by Thomas and Hopfield it was assumed that these states must 
exist based on the Zeeman experiments, but they had no direct evidence. 

Henry, Nassua, and Shiever studied the impurity doping of CdS and 
showed that Na and Li are the only shallow acceptors that can act as 
substitutional impurities . These shallow acceptors give rise to the 



36 

ll bound exciton that was described by Hopfield and Thomas. Usually 
two Ii lines are observed, and by varying the doping level these 
authors proved the lines to only be due to Na and Li. High purity 
crystals grown in clean reactor tubes were found to only exhibit the 
I^(Li) line. VThen Na was added a considerable broadening of the li 
line occurred, but as successive runs were made in the same tube, these 
authors showed the Na line could be resolved. Doping with K, Rb, or 
Cs, only resulted in a sharp I]^(Li) line, and P was found to give a 
complex shallow acceptor. The identity of the shallow donor level 
responsible for the I2 bound exciton could not be determined; however, 
the authors showed by donor-acceptor pair line splitting that the donor 
was not a native double donor such as a cadmixim vacancy, or a sulfur 
interstitial. The authors reasoned that the donor may be Na or Li 
interstitials because of the small size of these atoms, and when 
crystals are heavily doped, they become highly compensated. 
Unfortunately they were unable to prove the identity of the donor. 

Later work by Henry and Nassau involved measuring the 
spectroscopic lifetimes of the two bound exciton complexes . They 
were basing their work on another study by Thomas and Hopfield which 
showed the measured oscillator strength of the bound excitons to be 
very large, which meant that the radiative decay of the weakly bound 
exciton would be very fast. Thomas and Hopfield determined an 
oscillator strength of 9 +/-2 corresponding to a radiative lifetime of 
O.A +/-0.1 nsec.^-^ This oscillator strength is about 10^ greater than 
the strength of a free exciton. Henry and Nassau with their 
experimental setup were able to measure a radiative lifetime for the I2 



37 

exciton to be 0.5+/ -0.1 nsec. The very fast decay time of this state 
is what makes CdS so attractive for nonlinear optical application. 



Resonant Raman Scattering in CdS 

Further studies of exciton levels in CdS have involved the 
measurement of multiple phonon scattering from exciton states. Leite, 
Scott, and Damen found that the scattering of longitudinal optical (LO) 
phonons was enhanced when the scattering-phonon frequency coincided 
with that of excitons.^^ They were able to show up to nine orders of 
resonant Raman scattering occurring at frequencies shifted less than 1% 
from multiples of the 305 cm~^ line (the first LO line). These authors 
were not able to determine the identity of the exciton state (i.e. free 
or bound) which was acting as the intermediate state for the resonant 
scattering in this study, butin a latter paper by Leite, Scott, and 
Damen free excitons were proven to be the primary intermediate state. ■'-^ 
They also showed that bound excitons participated as intermediates by 
observing phonon sideband features on the photoluminescence of the 
bound excitons. The I^^ exciton was found to be a stronger resonant 
state compared to the I2 bound exciton. 

In a more recent paper by Mashshenko,^^ the temperature dependence 
of the LO and 2L0 lines associated with the A exciton were 
investigated. At 77° K the A-LO phonon predominates the emission, but 
as the temperature is increased to 110° K, this line decreases and the 
A-2L0 line increases. This increase in the 2L0 line indicates that an 
increase in the probability of a two-phonon process occurs at high 
temperatures . 



38 

Nonlinear Susceptibility of CdS 

Even with the obvious advantages of using CdS for nonlinear 
optical applications, very few groups have investigated this material. 
The primary amount of work in this area on CdS has been carried out by 
Dagenais.-^'^' As would be expected from the large oscillator 
strength of the bound exciton, a very large nonlinear refractive index 
results when this level is saturated, and the decay time is very short. 
By use of a Fabry-Perot arrangement and a narrow bandpass tunable dye 
laser, Dagenais reported a cw saturation intensity for this level of 
only 58 W/cm^, which corresponds to a nonlinear index of refraction of 
1 X 10"^ cm^/W.-^ In a later publication, Dagenais and Sharfin^^ report 
that by using a high finesse Fabry-Perot, a saturation intensity of 
only 26 W/ cm^ is required, which for the experimental setup corresponds 
to a nonlinear refractive index of 2 X 10"^ cm^/W. They also reported 
a switch up and switch down time of one and two nanoseconds 
respectively. These are the largest values of a nonlinear refractive 
index ever to be reported. 

Other work on CdS has been done by Bohnert, Kalt, and 
Klingshirn. They did not, however, study nonlinear ity by exciton 
saturation, but rather by the formation of an electron-hole plasma. 
High intensity laser pulses of energies just above the band gap energy 
were used to study this effect. By measuring the temporal line shape 
of the transmitted laser pulse they were able to determine the 
renormalization of the band gap due to the formation of an electron- 
hole plasma. Because this is a much smaller effect, intensities of 120 
kW/cm^ were required to produce a change in the transmitted pulse. 



39 

CdS Thin Films 

Vacuvim Deposition 

A great nximber of studies have investigated the thermal 
evaporation of CdS for the deposition of thin films. Although all thin 
films for this study were deposited by RF-magnetron sputtering, some of 
the results of these investigations are relevant to the present work. 
Many of the studies of thermal evaporation were undertaken to research 
the electrical properties of CdS thin films, although some optical 
properties have been studied. The problem is that most of these 
studies present results which both show differences from single crystal 
results and also differ from each other. The reasons for the 
discrepancies are related to the difficulties in evaporating CdS, which 
lead to problems with maintaining stoichiometry and crystal structure 
in the deposited films. The difficulty in evaporating CdS, as well as 
other chalcogenide compounds, is that complete dissociation of the 
compound occurs during evaporation.^^ If too high a temperature is 
used, then the compound will dissociate incongruently and because Cd 
has a higher volatility than S, non-stoichiometric thin films result. 
Source temperatures between 650° and 700° must be accurately controlled 
to avoid excess cadmium. ' Due to differences in the sticking 
coefficients of Cd and S, a non-stoichiometric thin film will also 
result if the an improper substrate temperature is used. Cook and 
Christy^' have found that if fused quartz substrates are used at 
temperatures greater than 200° C, then non-uniform films result. In 
comparison, Wohlgemuth et al.^^ have found that if the substrate is 



AO 

cooled to LN2 temperatures, then an amorphous film results which is Cd 
rich. 

The optical properties of CdS thin films are central to this 
study; however, the reported results for vacuum vapor deposited thin 
films are found to vary greatly. Most notably is the variation in the 
reported optical band gap and absorption coefficient, and the 
calculated index of refraction. Some of these differences can be 
realized by examining Figure 8 and Figure 9 which show the refractive 
index and absorption coefficient obtained by several authors. 
Referring to Figure 8 the results for single crystal CdS obtained by 
Cardona and Harbeke-^^ are given by curve a. Curve b is for 
polycrystalline film deposited onto fused silica at 180° C by Khawaja 
and Tomlin.-^^ Curve c corresponds to a polycrystalline film also 
deposited on silica at 180° C by Wohlgemuth et al.^^ and curve d is for 
an amorphous film deposited at LN2 temperatures by Wohlgemuth et al. 
The same authors correspond to the same curve letters of the reported 
absorption coefficients shown in Figure 9. As shown in Figure 8 the 
results by Khawaja and Tomlin show the closest resemblance to single 
crystal results, while the Wohlgemuth et al. results show a marked 
difference. In contrast, as shown in Figure 9, the absorption 
coefficient of polycrystalline films deposited by Wohlgemuth et al. 
model single crystal results best. The highest quality films in these 
studies were deposited at high temperatures, although a more recent 
study by Cook and Christy^^ reports similar results for room 
temperature depositions. It is obvious that subtle differences in the 
deposition technique result in a wide variation of thin film 



Al 




Figure 8 Variation of index of refraction reported by several 
authors. 



42 




Figure 9 Variation of the absorption coefficient reported by several 
authors . 



43 

properties. This is probably due to differences in thin film 
microstructures , which in the above studies were only determined by X- 
ray diffraction. No direct measurements (i.e. electron microscopy) 
were used to examine thin films. 

Although some of the optical properties of evaporated thin films 
have been investigated, albeit there are differences in the results, 
apparently very few studies have been made of the photoluminescence of 
thin films produced by this technique. Christmann et al. describe 
evaporation deposited epitaxial CdS films which displayed both green 
and blue edge luminescence. This is one of the first studies to relate 
both the morphology and composition of thin films to the observed 
photoluminescence. Thin films were deposited on cleaved surfaces of 
SrF2 at temperatures from 210° to 310° C. Variation of the 
supersaturation by controlling either the source temperature or the 
substrate temperature resulted in three basic morphologies. At low 
supersaturations, smooth CdS thin films were produced which only 
displayed broad band green photoluminescence. By decreasing the 
substrate temperature or increasing the source temperature, films were 
produced which displayed a structure with many hexagonal flat tops. 
These films showed green edge emissions and very low intensity blue 
edge emissions. Finally, at very high supersaturations, films which 
displayed a morphology with many hexagonal pyramids were found to show 
very intense blue edge, or bound exciton photolximinescence. 

Through cathodoluminescence and microprobe studies it was 
determined that the hexagonal pyramids were cadmium rich, and they did 
not contribute to the Iximinescence. Only the areas adjacent to the 



A4 

hexagonal pyramids were fotind to show bound exciton luminescence. The 
pyramids were thought to have nucleated from Cd droplets, and as they 
grew, the adjacent areas were depleted of cadmium. The required donor 
states for I2 bound excitons were therefore provided by cadmium 
vacancies. Other films were found to be uniformly slightly cadmium 
rich, and it was thought that cadmium interstitials were responsible 
for the broad band photoluminescence. 

Humenberger et al.-^-^ also reported thin films which displayed 
bound exciton Itiminescence. These films, however, were made by a hot- 
wall epitaxial technique, which is similar to the vapor phase technique 
used to make bulk single crystal platelets. Films were deposited on 
BaF2 substrates. The surface morphology was found to be smooth with a 
low density of hexagonal flat tops. No compositional data were given 
for thin films; however, acceptor states were provided by indium doping 
thin films during the deposition process. Free carrier concentrations 
as a result of the indium doping were measured to be on the order of 
10^7 to 10^8 cm-3.33 

The above two studies are the first to show by photoluminescence 
the presence of exciton levels in thin films. Studies of several other 
thin film deposition techniques such as chemical bath,-^^ or spray 
pyrolisis-'-' have reported photoluminescence, but none have shown 
exciton emissions. To see these transitions, it is apparent that very 
high quality thin films are required, and it is obvious that standard 
vacuum techniques or other techniques such as those described here are 
not capable of producing thin films of the necessary degree of quality. 
A deposition technique which permits greater control over the 



45 

deposition process is RF sputtering, and several investigations of the 
deposition of CdS by this technique are presented below. 

RF Sputtering 

Only recently has the deposition of CdS thin films by RF 
sputtering been investigated to any great extent. One of the first 
investigations was reported by Lagnado and Lichtensteiger . They 
described some of the properties of CdS thin films produced by RF-diode 
sputtering. A very strong preferred orientation of the CdS 
crystallites was found to occur with the c-axis parallel to the 
substrate plane. Electrical resistivity measurements at different 
temperatures revealed two activation energies for conduction, 
interpreted to show the activation of an unspecified trap below the 
conduction band, although no data on the chemical analysis for 
impurities was presented. 

Recently the technique of RF-diode sputtering has received 
considerable attention for producing CdS thin films for solar cell 
heterojunctions . The large photoconductivity of CdS makes it an 
attractive material for both solar cell and sensitive photodetector 
applications. A number of investigations on the RF diode-sputtering of 
CdS for these applications have been reported by Martil et al.-^^"^-'- 
These workers have investigated the dependence of the physical, 
electrical, and optical properties on both deposition parameters and 
post deposition heat treatments. 

The earliest publication by Martil et al.-^^ outlines the 
dependence of deposition rate, thin film grain size and resistivity as 



46 

a function of sputtering power, pressure, substrate temperature and 
substrate bias. Thin films were deposited onto fused silica 
substrates. The first effect they describe is the dependence of the 
deposition rate and resistivity on the pressure of gas used. A maximum 
in the deposition rate and a minimtim in resistivity occurred for a 
sputtering pressure of 5 Mm (5 X 10"-^ torr). For higher pressures they 
foxind the rate actually decreased, and the resistivity increased. The 
decrease in deposition rate at higher pressures was explained by a 
backscattering mechanism which increases the probability that more 
sputtered atoms will return to the target at higher pressures. The 
minimiim in resistivity at 5 um pressure was claimed to be a result of 
the maximum deposition rate that occurred at this pressure. Although 
no direct proof was given, the authors claim the high deposition rate 
decreased the number or concentration of impurities that were trapped 
in the film. 

The second effect described in this early paper by Martil et al.-^^ 
was the dependence of thin film grain size on the substrate temperature 
and substrate bias. The grain size was determined by SEM observations. 
As would be expected, the grain size of films increased from 300 A to 
3500 A as the substrate temperature was increased from 90° C to 300° C. 
The deposition rate accordingly decreased as the temperature was 
increased. When the substrate bias was increased to above -100 volts, 
the resulting thin films were found to be amorphous. The temperature 
effects can be explained by considering that with increasing 
temperature 1) the critical size for a nucleus increases, 2) the 
surface diffusion coefficient increases and 3) the sticking coefficient 



47 

for a material decreases. The appearance of an amorphous structure 
with high substrate bias was explained by the authors as a result of 
structural damage that occurred due to ion bombardment (which was 
increased by the high negative bias). 

The second report published by Martil et al.-^^ further explored 
the effects of substrate temperature and bias on the electrical 
properties of CdS thin films. They found that the resistivity 
increased from 10 to 10^ a cm as the temperature was increased from 60° 
C to 250° C, which they claimed was due to a change in stoichiometry 
(i.e. a loss of cadmium with increasing temperatures), although no 
chemical analysis was presented to back up this claim. A minimum in 
resistivity was also found with a substrate bias of -50 volts, or with 
a floating substrate (which developed a self bias of -28 volts). This 
effect was explained by the ion bombardment that results from these 
bias voltages preferentially resputting oxygen and other impurities 
from the growing film. Oxygen has been shown to be a acceptor- like 
trapping center, located 0.9 eV below the conduction 

band. '^2 

ain no 

chemical analysis was reported to prove that a decrease in oxygen 
occurs. Also the variation in activation energy for conduction that 
occurs with temperature as seen by Lagnado and Lichtensteiger^^ was not 
explored by these authors . 

The third publication by Martil et al.'^^ reports the influence of 
the above sputtering parameters on the optical properties of thin 
films. The structural changes which occur under different conditions 
were also further explored. Increasing the substrate temperature 
resulted in similar structural changes to those reported in the earlier 



48 

study; the grain size varied between 500 A and 3000-4000 A as the 
substrate temperature was increased from 60° to 300° C. This report, 
however described the sputtering pressure dependence of the 
crytstallinity. The sputtering pressure determines how much structural 
damage occurs due to ion bombardment. At low pressures, bombardment is 
enhanced by a large self-bias that develops on the substrate, and an 
amorphous structure results as previously described. At higher 
pressures, the crystallinity was also found to decrease, which was 
explained by the authors as due to a porous structure that develops 
from trapped gases. 

Optical properties were found to be a function of both substrate 
bias and sputtering pressure. A maximum in the optical band gap (2.36 
eV) was found to occur for a floating substrate bias, whereas the 
minimum in the band gap (2.30 eV) occurred for a -110 volt bias. The 
band gap was found to increase with an increase in sputtering pressure 
and became nearly constant for pressures above 10 pm. The pressure 
dependence of the refractive index however contradicts the pressure 
dependence of the band gap. The index of refraction was found to be a 
maximum with 5 pm pressure, and it decreased with increasing pressure. 
This was explained in terms of the porous structure which occurred at 
higher pressures; however, if the refractive index is reduced at higher 
sputtering pressures due to a more porous, less crystalline structure, 
then the band gap energy should also decrease. An increase in the 
nxamber of crystalline defects should cause tailing of the band gap, 
which is seen as a decrease in the gap energy. 



A9 

The latest investigation to be reported by Martil et al.^^ 
describes the effects of heat treatments on the electrical and optical 
properties of sputtered films. Heat treatments were carried out under 
H2 and N2 atmospheres at temperatures ranging from 100° C to 550° C and 
for times ranging from 20 minutes to 5 hours. The primary effect on 
the electrical properties was a two order of magnitude reduction in the 
resistivity to 4 X 10"^ cm for heat treatments at 200° C. Carrier 
mobilities accordingly increased to 50 - 70 cm^/Vs for this treatment. 
Higher temperatures were found to increase the resistivity and decrease 
the mobility. The decrease in resistivity was explained by claiming 
that oxygen desorption from the grain boundaries occurs at 200° C. As 
previously described, oxygen is a trapping center which in this case 
decreased the carrier concentration and simultaneously increased the 
scattering which reduces the mobility . No explanation was given for 
the increase in resistivity with increasing temperature, although it 
could be due to a loss of cadmium. 

The optical absorption edge was shown to become sharper and occur 
at a higher energy for heat treatments at 200° C. Temperatures higher 
than this were not reported to significantly alter the absorption edge; 
however, treatments above 550° C produced an overall decrease in the 
transmission. This was explained by a dissociation or reevaporation 
process that occurred as a consequence of the higher temperatures. The 
optical band gap was found to increase from 2.36 eV to 2.39 eV for 
treatments at 200° C, which was again explained in terms of oxygen 
desorption from the grain boundaries, although no chemical analysis was 
reported to prove this hypothesis. 



50 

Semiconductor Doped Filter Glasses 

Sharp cutoff filter glasses are silica based glasses which contain 
a fine dispersion of semiconductor microcrystallites. The variation of 
the sharp absorption edge position is achieved by either varying the 
composition and or heat treatment schedule. Great interest has been 
generated recently because it is thought by many authors that the 
crystal size developed in these materials by thermal treatments is on 
the size order for quantum confinement effects to occur. For glasses 
containing a mixture of CdS and CdSe many authors have reported a blue 
shift in the absorption band edge. In addition, a blue shift of the 
high energy exciton photo luminescence peak has been interpreted in 
terms of quantiim confinement. 

Warnock and Awashalom^-^'^^ in two publications on mixed crystal 
glass (those containing CdS 27/Se 73) report that the glasses which 
displayed smallest size distribution (average size = 94 A) exhibited 
the largest shift in the exciton photoluminescence, and this 
photoluminescence was shown to decay on a time scale of only 18 psec, 
which is nearly two orders of magnitude faster than the lifetime 
displayed by excitons in bulk material. The shifts in peak position 
and fast lifetime were interpreted in terms of a quantum confinement 
effects. 

A very recent paper by Borrelli et al.^^ has shown that the blue 
shift in the photoluminescence displayed by this particular mixed 
crystal glass is not due to confinement effects, but rather to 
compositional effects. A careful study of the crystal structure by X- 
ray diffraction revealed that a change in the stoichiometry occurs 



51 

during the heat treatments which develop the microcrystallites . It was 
postulated by these authors that more of the selenium remains in glass, 
while sulfur is more easily incorporated in the crystallites.^^ 
Therefore, at the lower temperatures which produce the smaller 
crystallites, the crystallites end up containing more sulfur, so the 
optical properties of glasses containing these crystallites are blue 
shifted toward the optical properties of pure CdS. True quantum 
confinement effects were, however, shown to occur in a series of 
experimental glasses which contained either CdS or CdSe, but not both. 



CHAPTER IV 
EXPERIMENTAL METHOD 



Vacuxim Deposition System 

Thin films for this study were made with a specially designed 
vacuum system. A schematic of the deposition chamber is shown in 
Figure 10, and a picture of the complete system is shown in Figure 11. 
The deposition chamber consists of a rectangularly shaped stainless 
steel box which measures 12" X 18" X 18". This chamber was custom 
built by MDC Vacuum Corp. (Haywood, CA.). Numerous ports and 
feedthroughs on the chamber permit a wide variation of deposition 
configurations. The vacuum pumping system utilizes a 330 1/sec 
turbomolecular pump (Balzers, Hudson, NH), a molecular sieve trap (MDC 
Vacuum Corp.), and a 300 1/min two-stage mechanical pump (Sargent 
Welch, Skokie, IL). With the molecular sieve trap activated, pressures 
in the high 10"^ torr range were possible using the mechanical pump 
only. Pumpdown from atmosphere pressure to 1X10"^ torr could be 
accomplished in 2 hours with use of the turboptimp. Vacuxjm gauging was 
performed with a Leybold Heraues (East Syracuse, NY) model CM 330 
combined Penning discharge and thermocouple gauge controller. 

Pressures for sputter deposition were controlled by using a 
micrometer adjustable throttle valve (Sputtered Films Inc. Santa 
Barbara, CA.) and a mass flow meter/controller (Matheson Gas Products 
Norcross, GA). High purity argon gas (99.9995%) was used as the 

52 



53 



CRYSTAL 

THICKNESS 

MONITOR 



RF MAGNETRON 
SPUTTER 
GUN 



p-l GAS INLET 
I MASS FLOW METER 



LN2 COOLED 
SUBSTRATE HOLDER 




o 
o 



CRYOSHEILD 



SHUTTER 



THROTTLE VALVE 



CAPACITANCE 
MANOMETER 



a RESIDUAL 
a GAS 
a ANALYZER 



r 



liMiil""""""""""""inn 

TURBOMOLECULAR PUMP 

lllNllli llliil 
llililllllllllliililll' 



Figure 10 Schematic diagram of sputter deposition chamber. Computer 
figure shown to indicate computer control of system. 



54 




Figure 11 Photograph of complete deposition system showing 
sputtering chamber and instrumentation. 



55 

sputtering gas. Initially the thermocouple vacuum gauge was calibrated 
with a capacitance manometer (MKS Instruments Inc. Burlington, MA) for 
different settings of the throttle valve and mass flow controller. 
Based on the previous work on CdS sputtered films, a pressure of five 
microns (5X10"-^ torr) was chosen and the system was calibrated for this 
pressure. To obtain this pressure an argon flow rate of 20 cc/m was 
needed. 

Film thickness during deposition was monitored with a Leybold- 
Heraeus IC 6000 deposition controller. This instrximent utilizes a 
quartz crystal oscillator microbalance for determination of film 
thickness. All parameters of calibration and deposition control are 
software programmable with this unit, either via the front panel 
controls, or through an RS-232 communications link to an external 
computer. The IC 6000 permits monitoring of up to six different films, 
and by using a sample and hold program, two different films can be 
monitored and controlled simultaneously. Close loop control of 
deposition rates was achieved by connecting the IC 6000 thickness 
monitor to the RF power supplies. 

For semi-automatic control and constant monitoring of the 
deposition process a Zenith Z-158 personal computer with 512K RAM and a 
20 megabyte hard disk was interfaced to the system. A Dascon 1 I/O 
board (Metrabyte Corp.) which provides four channels of analog and 12 
bits of digital input/output was installed in the computer. With this 
interface board and with RS-232 communications to the IC 6000 
deposition controller, it was possible to monitor nearly every aspect 
of the deposition process. The RS-232 communications permitted direct 



56 

programming control over deposition parameters and constant monitoring 
of deposition rate, thickness, power output and crystal oscillator 
status. The analog portion of the I/O board was connected to the 
pressure gauge controller and to the RF power supplies. Connection to 
the power supplies permitted recording of the DC bias which develops on 
the target as a result of the sputtering process. All of these process 
parameters were stored on disk during a deposition run. This made it 
possible to go back and review the deposition process, if a film was 
later found to have anomalous properties. 

A unique feature of the deposition system has to do with the 
various substrate holders which could be used. For low temperature 
depositions, a liquid nitrogen (LN2) cooled substrate holder was 
employed. This holder is made up of two double concentric tubes in 
which the center tube acts as a reservoir for LN2 and the outer tube, 
which is open at the bottom to the vacuum system, acts as an insulating 
vacuum jacket. An aluminum plate with a slightly recessed area to hold 
the substrate is attached to the end of the inner reservoir tube with a 
copper screw. To reduce the amoxint of contamination that would occur 
on the cold substrate, an additional LN2 cooled coil was positioned 
around the substrate holder assembly. This coil is referred to as the 
cryoshield in Figure 10. For room temperature depositions, the same 
holder was used, however, without filling the LN2 reservoir. 

High temperature depositions were accomplished by replacing the 
LN2 feedthrough assembly with a resistively heated substrate holder. 
This holder was made by drilling holes lengthwise in a thin aluminum 
block and inserting nichrome wound ceramic heaters. A Chromel-Alumel 



57 

thermocouple was embedded in the core of the holder, and control of the 
temperature was made by controlling the input power to the holder with 
a variable autotransf ormer . Substrates could be heated to 300° C with 
this arrangement with a temperature control of +/- 5° C. 

A single planar magnetron sputter gun supplied by US Guns 
(Campbell, CA) was used for sputter depositing the CdS thin films. The 
gun accepts two inch diameter targets, which can range from l/16th to a 
quarter of an inch in thickness. An Eratron RF power supply 
(Campbell, CA) operating at 13.57 MHz with a power capability of 600 
watts was used to supply RF power to the gun, through an auto load 
match tuning network. The matching network is require to balance the 
impedance of the gun and plasma to the 50 ohm output from the power 
supply. If the impedance is not matched, then the RF power is 
reflected, and the only thing that is accomplished is heating of the 
heat sinks in the power supply. The utility of an auto load network is 
that when conditions of the plasma change, the network will 
automatically match the impedance, thereby eliminating any reflected RF 
power . 

High purity cadmium sulfide sputtering targets were obtained from 
CVD Industries (Woburn, MA). These targets are made by a chemical 
vapor deposition process so they are supplied with a bulk density of 
nearly 98% of theoretical density. High bulk density reduces the 
amount of outgassing during sputtering. All impurities were less than 
1 ppm and the stoichiometry of the target material was slightly cadmium 
rich (50.46%). Nearly all thin films for this study were made from CVD 
targets; however, a different type of sputtering target was obtained 



58 

from EM Chemicals Inc. (Hawthorne, NY) for testing. This target was 
made by a hot pressing technique, which results in a lower bulk density 
and a higher impurity content. Impurity information was not given by 
the company for these targets; however, thin films that were sputtered 
from these targets were found to contain much higher levels of iron 
than the levels found in films sputtered from CVD targets. 

Targets that were used for COSAD were made from a piece of glass 
obtained from Schott Glass Industries (Duryea, PA). The glass was a 
special type of borosilicate crown glass (#8329) which is similar in 
composition to a BK-7 glass, but it is processed in such a way so that 
all the volatile impurities in the glass are removed. This glass is 
sold by Schott as a special type of over coating glass used for thermal 
evaporation. Targets were made by first sectioning the as supplied 
cylinder of glass into 3/16 inch discs from which two inch blanks were 
cut. Both sides of a blank were polished to 600 grit before being used 
as a sputtering target. After it was found that a high power level was 
required to sputter this glass, a l/16th inch copper plate was bonded 
to one blank to improve thermal contact to the sputter gun. The 
bonding was accomplished with an indium tin solder. 

For producing COSAD films, the deposition chamber was reconfigured 
to accept a second RF planar magnetron sputter gun, for sputtering the 
glass target. This gun was used with a second 600 watt Eratron supply 
and the two power supplies were driven by a common oscillator to phase 
match the two plasmas. In addition, a set of baffling plates were 
mounted inside the chamber as shown in Figure 12. The center baffle 
plate was used to isolate the two plasmas. A pair of three inch 



59 



ROTATION 
FEEDTHROUGH 



CRYSTAL 

THICKNESS 

MONITOR 




SUBSTRATE HOLDERS 



SHUTTER 



SHUTTER 



BAFFLE 
PLATE 



BK-7 GLASS 
SPUHER GUN 



Figure 12 Schematic diagram of deposition chamber configured for 
COSAD. 



60 

diameter apertures centered over the two sputter guns were cut in the 
top plate to provide access to the two plasma sources. A dual 
substrate holder was rotated in the plane above the two apertures, so 
that the substrate was alternatingly exposed to one source and then the 
other. The dual holder provided the capability of producing two films 
during a single deposition run. Initially a DC motor with a reducing 
gear drive was used to rotate the substrates with a linear speed. A 
more sophisticated direct drive stepping motor was later added to the 
system to permit variable exposure to the two plasma sources. The 
stepping motor was interfaced with the Dascon I/O board in the Z-158 
computer via an external digital control circuit. 

One disadvantage of the COSAD configuration was that substrates 
could not be heated or cooled and the temperature of the substrate 
could not be monitored during a deposition. The utility of this 
particular system configuration, however, is that single component 
films could still be produced, simply by positioning the substrate 
holders over the respective apertures, and exposing the substrate to 
deposit a film. 

RF Magnetron Sputtering 
As described in the Bibliographic Review section, the only sputter 
deposition of CdS was carried out by RF-diode sputtering. In this 
configuration two parallel plates are used. The target is attached to 
the cathode plate, and substrate is attached to the anode plate, which 
can either be at ground or floating potential. A plasma is generated 
between the two plates by applying RF power to the cathode plate. 



61 

Electrons in the plasma can be accelerated by the positive half of the 
RF field, which results in both the ionization of argon atoms, and the 
bombardment of the target. This bombardment results in the development 
of a negative potential on the target surface, which is called the DC 
bias. The argon ions in the plasma are too massive to be accelerated by 
the RF field; however, they are accelerated towards the target by the 
DC bias, which results in sputtering of the target material. 

The configuration of diode sputtering results in exposing the 
substrate and growing films to the full energy of the plasma. The 
growing film is continuously bombarded by a number of energetic 
particles, both charged and neutral. This bombardment can either be 
enhanced or reduced by applying a potential to the substrate during 
deposition. Substrate biasing can only reduce the amount of 
bombardment to a certain extent, essentially because the substrate is 
totally emersed in the plasma during deposition. Considerable heating 
of the substrate and film occurs as a result of the bombardment, even 
at moderate plasma powers. 

RF planar magnetron sputtering reduces the amount of bombardment 
by incorporating a strong permanent magnet behind the cathode assembly. 
The configuration of a US sputter gun is shown schematically in Figure 
13. Curved magnetic field lines force the electrons in the plasma into 
circular orbits, which both enhances the plasma immediately above the 
target, and reduces the amount of electron bombardment on the growing 
film. The target is a circular disk that is secured to the magnet 
housing, which comprises the cathode. The anode is comprised of a 
circular cap (called the ground shield in the Figure) which fits over 



62 



GROUND 




Figure 13 Cross-sectional view of RF magnetron sputter gun made by US 
Guns Inc/^ 



63 

the gun assembly. Most of the plasma is constrained to the area 
immediately above the target, so a large reduction in substrate 
bombardment and substrate heating is realized. The substrate and 
growing film, however, can still be bombarded by energetic neutral 
particles, which has been shown to be a major cause of substrate 
heating. Still the heating is reduced and as will be pointed out in 
the Results section (Chapter 5), a considerable difference in the as 
deposited film properties occurs with RF planar magnetron sputtering, 
compared to RF diode sputtering. 

Thin Film Deposition 

Prior to deposition of thin films, all substrates were subjected 
to a three phase cleaning procedure. The first phase of the process 
was an ultrasonic bath in DI water for 10 minutes. After this 
treatment the substrates were removed from the water and rinsed with 
isopropyl alcohol (IPA). A second ultrasonic cleaning was then carried 
out for 10 minutes in IPA. The final phase of the cleaning procedure 
was a 15 minute treatment in a vapor degreaser, which employed IPA as 
the solvent. Substrates were slowly removed from the vapor, which 
allowed the condensed vapor droplets to evaporate. To check for 
cleanliness substrates were examined by edge illumination against a 
black background. This permitted observation of any contaminates on 
the surface of the substrate in addition to any small flaws in the 
surface. 

To deposit thin films, substrates were mounted on a substrate 
holder and positioned in the deposition system immediately after 



64 

cleaning. The deposition system was pumped down to a pressure of < 1.0 
X 10"^ torr before backfilling with the sputtering gas. The IC-6000 
was usually programmed for a fast ramp up to the desired power level 
and then a rate controlled presputter was initiated before the shutter 
was opened. If a sputtering target was newly installed, a presputter 
burn- in of 30 minutes was carried out; otherwise the target was 
presputtered for 5 minutes before exposing the substrate to the plasma. 
The IC-6000 automatically controlled the deposition rate and thickness. 
Deposition rate was varied between 1 to 5 A/sec and films for optical 
analysis were typically made 1 pm thick, although the film thickness 
for other analysis was varied. 

Substrate materials 

Several different types of substrates were used to support thin 
films, depending on how the films were to be analyzed. Standard 
substrates for optical analysis were made of optical quality fused 
silica, typically one inch square and 1 mm thick. These substrates 
were found to have very few surface flaws and were essentially 
transparent over the optical region in which films were analyzed. The 
low index of refraction (n=1.46) of these substrates was also necessary 
for measuring the refractive index of the film and for planar waveguide 
measurements. Some films were also deposited on silica for TEM 
analysis. Self-supporting TEM specimens were prepared by floating 
these films off the silica with a 1% HP solution. 

Other films for optical analysis were deposited onto single 
crystal sapphire substrates. This was done to determine if the high 



65 

thermal conductivity of the sapphire would aid in reducing heating by 
the laser beam during low temperature spectroscopic measurements. 
These substrates were of very high quality with no detectable flaws and 
a very smooth surface finish. Some COSAD films were deposited onto 
sapphire substrates to aid in X-ray chemical analysis, since the 
sapphire had no X-ray lines which overlapped the lines in these films. 

For other TEM analysis, films were deposited on slabs of single 
crystal NaCl and on TEM grids which held carbon support films. The TEM 
grid-carbon film combination was used for quick analysis of deposited 
films. Once the grid-carbon film assembly was prepared, thin films 
could be deposited directly onto the carbon film, and no other sample 
preparation was necessary. Films of thickness from 500 to 3000 A were 
typically deposited for TEM analysis. The carbon support film was made 
by thermally evaporating approximately 200 A of amorphous carbon onto 
sheets of single crystal mica. After the carbon film was floated off 
the mica, 300 mesh TEM grids were carefully placed on the floating 
film. The film with grids was then picked up on a thin cardboard card 
and allowed to dry. 

Single crystal NaCl was used as a substrate because it was found 
that after heat treatment or as a result of a high temperature 
deposition, thin films of CdS could not be floated off silica 
substrates. Normally the 1% solution of HF would readily float films 
off silica, but after any type of heat treatment above 200° C not even 
a high concentration of HF was successful for floating the films. 
Apparently some type of chemical bond forms between CdS and the silica 
even at relatively low temperatures. Single crystal NaCl substrates 



66 

were also required for TEM analysis of COSAD films, since the HF 
solutions normally used to lift CdS off silica slides would strongly 
attack these films. 
Substrate temperatures 

Several different substrate temperatures were used to investigate 
the effect of deposition temperature on thin film grain size. With 
the LN2 feedthrough assembly that was described above, substrates could 
be cooled to LN2 temperatures in about 20 minutes. For some 
depositions a low temperature thermal contact paste was use to mount 
the substrate in the holder, although most frequently, standard hold 
down clips were used. An Iron-Constantan thermocouple was initially 
used to monitor the temperature of the substrate holder during a 
deposition. To use a digital meter to measure the output from this 
thermocouple during a plasma deposition, an RC circuit had to be used 
to decouple the RF signal that was imposed on the DC thermocouple 
signal. The RC circuit is essentially a low band pass filter. A 
schematic diagram of the circuit is shown in Figure 14. If this filter 
is not used, a very large RF signal can be induced in the lead wires, 
which will destroy most D/A converters used in digital meters. 

For depositing films at room temperature, the LN2 feedthrough 
assembly (without LN2 in the reservoir) was used to hold substrates. 
These depositions were very close to room temperature, since it was 
found that even with the highest deposition rates, the holder would 
only heat up to 35° C. When substrate temperatures greater than this 
were required, the LN2 assembly was replaced with the resistively 
heated holder. Temperatures as high as 300° C could be obtained in 



67 



Thermocouple 





25^lH 
/ 



1" 



Figure 14 Schematic diagram of RC decoupling circuit used protect 
digital equipment from RF transients . '^^ 



68 

about 10 minutes with this holder, although most depositions were made 
at 200° C. The unique feature of this holder is that it permitted 
outgassing of the substrates prior to deposition. This was done by 
heating to 200° C for 30 minutes in high vacuum. 

Deposition rates 

A variation in deposition rates was also studied to determine its 
effect on thin film properties. For pure films of CdS, rates from 1 
A/sec to 5 A/sec were used. These rates were monitored and controlled 
by the IC 6000 deposition controller. Figure 15 shows a graph of the 
sputtering rate versus power density for both CdS and BK-7 targets. As 
shown, the glass target used for making COSAD films is much more 
difficult to sputter, as an input of 275 watts (13.5 W/cm^) only 
resulted in a deposition rate of 2 A/ sec. Power levels greater than 
300 watts could only be used with the BK-7 glass target which had a 
copper backing plate bounded to the back of the target. Even at 500 
watts, however, a deposition rate of only 4 A/sec could be obtained. 
At this power level, considerable heating of the substrate was found to 
occur, even when the substrate was rotated during COSAD runs. 

Co-Sputter Alternating Deposition (COSAD) 

By using the dual sputter gun configuration described above, a 
very unique type of thin film could be produced. The acronym describes 
the process by which these films were made; by rotating the substrate 
above the two sputter guns, an alternating layer of glass and the CdS 
was deposited, which was repeated continuously. Initially the two guns 



69 




POWER DENSITY (W/cm2) 



Figure 15 Graph of sputtering rate versus input power density for CdS 
and BK-7 targets. 



70 

were set up to sputter deposit at a static rate of 3 A/ sec. This is 
the maximum rate that could be used with the glass target without 
causing excessive heating. The circle over which the substrate was 
rotated was eight inches in diameter, so with the three inch diameter 
apertures used with the guns, the duty cycle was rather low, only 0.12 
for each gun. With a rotation speed of 20 RPM, the resulting total 
dynamic deposition rate is less than 1 A/sec. One possible way to 
increase this rate would have been to reduce the rotation speed. This, 
however, would also mean that the substrate would be spending more time 
over areas which were not depositing film and were actually depositing 
impurities . 

The use of the computer controlled stepping motor to rotate the 
substrate holder alleviated the problem with the linear drive system. 
With this setup is was possible to increase the amount of time the 
substrate spent over a given source. At the same time, it reduced the 
amount of impurities introduced into the film because the stepping 
motor could be driven full speed between the two sources, which was 
about 120 RPM. By simply altering the computer program, this system 
permitted easy variation of deposited structure. 

Post Deposition Treatments 
To improve many of the physical properties of thin films a post 
deposition heat treatment had to be instituted. Generally, heat 
treatments on CdS thin films were made in a fused silica muffle tube 
furnace under a flowing argon atmosphere. The argon used for heat 
treatments was of the same grade as the gas used for sputtering. For 



71 

annealing COSAD films high purity compressed air was used with the same 
furnace setup. Flow rates of 100 cc/ra were typically used. 
Temperature control was accomplished with a single setpoint temperature 
controller (Love Control Co.) which provided temperature control to +/- 

1° C. Two thermocouples were used, one mounted outside the tube for 
control and one positioned inside the tube adjacent to the sample. 
Heating rates of up to 35° C/min could be realized, and cooling rates 
of 100° C/min could be obtained by sliding the muffle tube out of the 
furnace hot zone. Temperatures for heat treating thin films ranged 
from 200° C to 700° C, with times ranging from 1 minute up to 5 hours. 

To reduce the amount of grain growth that would occur as a result 
of high temperature heat treatments, some films were heated by a 
technique known as rapid thermal annealing. This process utilizes high 
power quartz lamps to rapidly heat the sample. Because of the low 
thermal mass that is achieved with this technique, heating rates in 
excess of 500° C/sec are possible. The sample is sandwiched between 
two thin sheets of graphite which act as black body absorbers and help 
to transfer the heat generated by the lamps to the sample. In this 
particular system lamps with a total power of 5000 watts were used. 
Heat treatments up to 650° C with times ranging from 10 to 60 seconds 
were done under a flowing nitrogen atmosphere. 

Thin Film Characterization 

Microstructure 

Determination of thin film microstructure was achieved using 
several analytical techniques. Direct examination of the 



72 

microstructure was made by electron microscopy. The various TEM 
specimens that were described in a previous section were examined in a 
JEOL 200 CX analytical microscope (Japanese Electron Optics Laboratory, 
Boston, MA.). This instrument was used in both transmission (TEM) and 
scanning transmission (STEM) modes to directly observe and measure the 
ultra-fine structure of thin films. Selected area diffraction (SAD) 
mode was used to conduct electron diffraction experiments for 
determination of crystal structure and orientation. In the SAD mode it 
was possible to obtain diffraction information from areas as small as 5 
um. The line to line resolution observed in the TEM mode was 8 A. 
Another technique used for determination of crystal orientation was 
dark field imaging. By tilting the primary beam in the microscope, the 
contrast that is observed in a dark field image is only due to those 
crystals or portions of crystals which are oriented for a specific 
Bragg reflection. Dark field imaging makes it possible to observe 
preferred orientation in a thin film or any portion of a crystal that 
contains defects (such as microtwins) . On the JEOL 200CX changing 
from bright field to dark field was accomplished with a single switch. 

To study the topographical structure of thicker films a JEOL 35CF 
scanning electron microscope was used. Films which were made for 
optical analysis were frequently examined with this instrument. The 
advantage of this technique for CdS films was that no sample 
preparation was required except for sample mounting. The conductivity 
of these films was high enough not to warrant coating with a conductive 
film, although for COSAD films a thin carbon film had to be deposited 
before these films could be examined. With this instrioment the 



73 

presence of gross defects in the film such as pinholes or cracks could 
be observed, as well as the fine grain structure of certain films. The 
major disadvantage with this microscope, however, was a limiting 
resolution of about 300 A which made it difficult to observe the grain 
size of as-deposited thin films. 

X-ray diffraction was another technique used to measure the 
crystal structure and orientation of thin films. An APD 3720 computer- 
controlled dif f ractometer (Phillips Co.) was used to carry out 
diffraction experiments. Typically, diffraction was measured from two- 
theta values ranging from 15° to 90° with a angular step of 0.05°. Use 
of the computer to store X-ray diffraction spectra on disk greatly 
facilitated the determination of the change in crystal structure with 
heat treatments. Changes in peak positions and intensities could be 
readily determined, and by making high resolution scans (0.02°/step) 
over certain peaks it was possible to calculate the grain size and film 
strain using certain utility programs available in the computer system. 
Again, no sample preparation was necessary for this technique as 
diffraction from 1 pm films supported on silica substrates was readily 
observed . 

Chemical Analysis 

Initially, Energy Dispersive Spectroscopy (EDS) was used to 
measure the stoichiometry of thin films during both SEM and STEM 
observations; however, it was found to give results that were not 
quantitative. Particularly in the SEM, it was found that the results 
were very dependent on sample geometry with respect to the detector. A 



74 

technique know as X-ray secondary fluorescence was used instead to 
measure the stoichioraetry of nearly every thin film that was made. The 
particular instrument used for these measurements utilized a silver X- 
ray tube to produce the primary X-ray beam. Tube voltages of 50 kV 
were used with a current of 30 mA. One of four different secondary 
targets could be selected to generate the X-rays that were used to 
analyze the sample. The advantage of this technique is that the 
background radiation from the X-ray tube (bremsstrahlung) is totally 
removed and only the very narrow X-ray line corresponding to the 
secondary target reaches the sample which results in a great reduction 
of the background counts. The particular geometry of this system 
limits the radiation to only one polarization which further reduces the 
background count. Detection levels for most transition metals was ~ 
0.1 ppm. Beam size incident on the sample is 1 cm in diameter and 
penetration depth is only a few microns which makes this technique 
ideally suited for measuring thin films supported by substrates. The 
X-rays emitted from the sample are analyzed by EDS with this system; 
however, due to the above arrangements, a much more quantitative 
determination could be made when the results were compared to a 
standard reference. 

The standard used, in this case, to determine the stoichiometry of 
thin films was a piece of target material. The stoichiometry of the 
target was verified by a wet chemical technique and electron probe 
microanalysis (EPMA) (JEOL 730 Superprobe). For solution analysis a 
portion of the target was dissolved in nitric acid and then this 
solution was diluted a given amount. The amount of Cd and S in 



75 

solution was determined by using an induction coupled plasma (ICP) 
spectrometer (Allied Analytical Systems, Waltham, MA). By comparing 
these amounts to those in standard solutions the stoichiometry was 
determined. For EPMA the stoichiometry of the target material was 
determined by use of standards and a ZAP or (^pz program. The ratio 
of the integrated peak intensities for Cd and S in the target was then 
determined by X-ray fluorescence. The ratio obtained from thin films 
was compared directly to this ratio to quantitatively determine the 
stoichiometry . 

Film thickness could be readily determined with this technique by 
measuring the intensity of the silicon K-alpha peak that was emitted by 
the substrate through the film. The intensity of this line is 
exponentially attenuated by the thickness of material that it passes 
through and can be related by the following equation: 

I = Iq exp"°^ (2.1) 

where Iq is the intensity from a bare substrate and t is the film 
thickness. From equation 2.1 a linear relationship between a function 
of the intensity, F(I), and t can be obtained by^^ 

F(I) = - logio ^ . (2.2) 

lo 

The intensity from three different films of known thickness (verified 
by prof ilometry) were measured and by plotting F(l) versus t, a master 
curve was generated. The thickness of an unknown film could then be 
found by solving the above equation for F(l) using the intensity from 



76 

the unknown. The x-coordinate on the curve for the unknown F(l) would 
give the thickness. Film thickness of up to 3 pm could accurately be 
determined with this technique. Changes in density or loss of material 
as a result of heat treatment could also be monitored with this 
technique. 

Several other analytical techniques were used to analyze the 
composition of thin films. In each case the results for thin films 
were compared to the results obtained from target material. The 
different techniques were used to both acquire information, which the 
specific technique is most suited for, and to cross-check the 
composition of thin films. Auger electron spectroscopy (AES Perkin- 
Elmer, Norwalk, CT) gave the least quantitative determination of the 
absolute composition, but the technique was very quantitative for 
determining differences between samples. X-ray photoelectron 
spectroscopy (XPS Kratos.UK) gave semi -quantitative results and also 
showed how the surface of thin films were altered by heat treatment, a 
result which was not detected with any other technique. 

Optical Measurements 
By far the most extensive measurements made on thin films were 
optical measurements. The availability of several spectrometers, 
monochromaters, lasers, and optical hardware greatly facilitated these 
measurements. The UV-VIS spectrophotometer used for many measurements 
was a Perkin-Elmer Lambda 9 (Norwalk, CT). Photoluminescence was 
measured with both a Princeton Applied Research Optical Multichannel 
Analyzer (Princeton, NJ), and an Instruments SA Raman spectrometer 



77 



(Metuchen, NJ) . A variety of lasers were used for different 
experiments, although most photoluminescence experiments were made with 
a Spectra-Physics model 2025 Argon Ion laser (Mountain View, CA) . Many 
measurements were made at both room temperature and at temperatures 
down to 9° K. Low temperatures were obtained with an Air Products 
(Allentown, PA) closed-cycle helium cold finger and digital temperature 
controller. Experiments carried out with these instrxaments will be 
extensively detailed in the following sections. 

Index of Refraction 

A very unique technique for measuring the dispersion or index of 
refraction as a function of wavelength was used on thin films. 
Normally the index of refraction of a material is related to the 
percent of transmitted light by the well known equation: 



where the reflection, R, is given by (n - l)^l{n +1)^. The situation 
is much more complicated, however, for a thin film supported on a 
substrate with a different refractive index. The reflection that 
occurs at the film/ substrate interface must be dealt with as well as 
the reflections that occur at the other interfaces. This leads to a 
more complicated expression: 



T = (1 - R)^ 
1 - R^ 



exp (-2at) 



exp (-at) 



(A.l) 



T 



(1 - Ri) (1 - R2) (1 - R3) exp (-at) 



(4.2) 



(1 - R2R3) { 1 - [ RiR2 + R1R3 (1 - R2)^ ] exp (-2at) } 



78 

where Rj^, R2, and R3 are the reflectivities of the air-film, film- 
substrate, and substrate-air interfaces, respectively.^^ The values of 
the Rl, R2, and R3 are given by the same equation stated above with the 
refractive indices of the three materials (air, film, and substrate) 
substituted for n, respectively. Even if the value of the absorption 
coefficient is known, the transmission must be measured to 0.5% to get 
a 1% accuracy in the refractive index, which is experimentally very 
difficult to do unless a specially designed spectrometer is used. 

When the refractive index of the film is significantly different 
from that of the substrate (as the case with CdS on silica) a very 
strong wavelength dependent interference effect occurs . The effect can 
be explained by examining Figure 16. When the incident light beam 
passes through the thin film and encounters the interface it will be 
reflected due to the differences in refractive indices. Depending on 
the thickness and the refractive index of the film (the product of 
these two values determines the optical path length) the reflected ray 
will either constructively or destructively combine with the incident 
beam. This is the same process that occurs in a Fabry-Perot 
interferometer described previously. However, instead of changing the 
physical distance of the cavity to change the optical path length, 
resonance is achieved by scanning the wavelength of incident light. In 
terms of the total transmission that is measured, a series of 
interference fringes is observed, which vary with wavelength. These 
fringes are known as fringes of equal chromatic order (FECO), and the 
wavelengths at which maxima and minima in transmission occur is given 
by the same equation used for resonance conditions in a Fabry-Perot 



79 




gure 16 Interference effects at thin film, substrate, and air 
interfaces . 



80 



m A = 2 n d (4.3) 

where m is the order number of the fringe, d is the thickness, and X is 
the wavelength. To take advantage of this effect to determine the 
index of refraction two assumptions must be made. The first is that 
the order number is not a discrete number but that it can vary 
continuously. Why this is important will be described shortly. The 
second assumption is that the dispersion of the refractive index can be 
described by the following equation 



where the square root of B gives the infinite wavelength refractive 
index and A and x are constants. 

To make use of this effect, a FECO spectrum is obtained at normal 
incidence. A typical spectra from a 2 urn CdS film is shown in Figure 
17a. Curves are drawn tangent to the maximum and minimum peaks and the 
values of the wavelength of the tangent points are determined. The 
sample is then rotated in the beam by 30°, resulting in an increase in 
the optical path length by a factor of l/(cos 30). The assumption 
that the order number is a continuous mathematical variable means that 
the normal incidence spectrum can be shifted to a shorter wavelength by 
increasing each order niomber by an amount 6m. -^^ This increase produces 
a shift in the FECO spectra shown in Figure 17b. Again, tangent curves 
are drawn and the tangent points to the interference spectrum are 
determined. The difference in the pairs of tangency points are then 



81 



z 

o 

w 

3 




WAVELENGTH (urn) 



a) 



100 



z 
o 
Si 

U) 

2 

VI 




l.-t 

WAVELENGTH (urn) 



b) 

Figure 17 Fringes of equal chromatic order (FECO) spectriim for 2.0 um 
thick film of CdS on a silica substrate, a) Normal 
incidence spectrum showing many orders; b) Expanded 
abscissa scale showing shift in FECO spectrum for 30° 
incidence . 



82 

used in an iterative calculation to determine the values of A, B, x and 
n. 

The problems associated with this technique involve precise 
placement of the sample at the focal point of the beam and accurate 
rotation of the sample about the axis which passes through the focal 
point. These problems were addressed by using a micrometer adjustable 
x-positioner and a micro-rotation stage, which are shown placed in the 
Lambda 9 spectrophotometer in Figure 18. Normal incidence to the film 
was determined by rotation of the stage a few degrees on either side of 
the initial zero and observing the shift in position of a given 
interference fringe. The true zero point was taken as the position 
which produced the midpoint between the two shifts. The focal point of 
the beam was then determined by observing the peak value of a given 
fringe as the sample holder was moved with the positioner. 

The most difficult part of this technique (except for writing the 
iteration program for the analysis) was the manual determination of the 
value of the tangency points from the FECO spectra. The wavelength of 
the tangent point must be known to a precision of 0.1 nm to obtain an 
accuracy of three decimal places in the refractive index. Wavelengths 
can be measured with the spectrometer to this precision, but this 
precision could not be obtained by manual measurement of the tangent 
point from the printout of the spectra. 

The problem was alleviated by interfacing the Lambda 9 to an IBM 
AT computer. Use of the computer greatly facilitated all aspects of 
using the spectrometer as spectra could be stored on disk and 



83 




Figure 18 Photograph of micro-positioner stage used for acquiring 

FECO spectrum, shown mounted in Lambda 9 spectrophotometer. 



84 

manipulated later. The BASIC program used to remotely program, run, 
and accumulate data from the Lambda 9 is listed in Appendix A. 

For the determination of the refractive index a particular 
software package available with the AT was found to be very useful. 
This software called is Asyst (McMillain Software Co.) and it is a very 
powerful mathematical analysis prograim. To determine the refractive 
index, the numbers representing the FECO spectra were imported into 
Asyst and stored in a two dimensional array. Using a very short 
program, the array was plotted, and the equations for the curves 
tangent to the maximum and minimum were fit. Next, arrays were 
generated to represent each of the tangent curves. Asyst could then be 
used to very easily determine the points where the three arrays 
intersected, with an accuracy of 0.1%. The intersection points were 
stored in a fourth array. This process was repeated for a 30° 
incidence spectra. The very long iterative BASIC program that was 
originally used to calculate the dispersion equation could be shortened 
considerably with utility functions available in Asyst. In fact, by 
manipulating the numbers as arrays, the calculation was much simpler 
and much faster. 

Even though the above process produced an accurate result, a 
second technique, ellipsometry , was used to verify these results. 
Ellipsometry is based on the change in polarization of a light beam 
when it is reflected from a surface. Generally a plane polarized light 
beam is made incident on the surface of a material at an angle of A5°. 
The resulting reflected beam is elliptically polarized and from the 



85 

analysis of this polarization the optical constants of the material can 
be determined. 

The problems associated with this technique mostly originated from 
the actual instrximent. The computer-controlled Gaertner ellipsometer 
(Chicago, XL) is the industry standard; however, the unit available for 
use in this study was a manual model which greatly reduces the capacity 
for quantitative analysis. The problem does not so much have to do 
with manually setting the polarizer and analyzer, but rather in reading 
the analog scale to determine the node points. This was a special 
problem on this unit because the meter was nonlinear; the highest 
sensitivity was between a reading of 2 an 3 on a scale of 1 to 5. In 
addition, the gain for the photocell had to be manually adjusted as a 
node point was found. The gain was also nonlinear and there was a flat 
spot at one end of the range. These two problems contributed to 
difficulties in reproducing the results; however, the greatest problem 
with the analysis had to do with the computer program used to calculate 
the values of n and t from psi and delta. Supplied by Gaertner, this 
program did not actually calculate the values of psi and delta, but 
instead it looked up the values in a table, based on the values of the 
two pairs of angles that were inputed. From the table values of psi 
and delta the program then calculated the refractive index and 
thickness by an iterative technique. One difficulty with the program 
occurred if the particular combination of angles inputed were outside 
values in the table. The program would hang up and the only solution 
was to boot the program out. This actually happened a surprisingly 
large number of times, even when similar samples were analyzed. To 



86 

alleviate the problems with this program, a graphical method was 
developed to calculate the refractive index and thickness from the set 
of angles measured with the ellipsometer . A brief description of this 
method is given in Appendix B. 

UV-VIS Absorption 

Measurement of the fundamental optical absorption edge in 
semiconductors permits direct determination of the nature of the band 
gap. Optical absorption properties of thin films were measured over a 
variety of temperatures with the Lambda 9 spectrophotometer operating 
in one of several modes. In the transmission mode a linear, zero to 
100% scale was used, which was useful measuring FECO spectra. A more 
useful mode for measuring the band edge, however, was to plot the 
transmitted light in terms of absorbance. Because the absorbance scale 
is the natural logarithm of reciprocal transmission, features of the 
band gap absorption above the band edge could be studied. In the 
transmission mode unless the scale was expanded these features would 
appear indistinct, because transmission above the band gap was 
typically less than 1% with the thickness of films usually studied. In 
the absorbance mode this 1% transmission would correspond to a value of 
4.6 on a possible absorbance scale of 6.0. Another advantage of using 
the absorbance scale is that the spectra obtained qualitatively 
reflected the shape of the true absorption curve. As explained earlier 
in this chapter, the transmission spectriim that is measured of a thin 
film on a substrate is the result of a complicated combination of the 
reflections which occur at the various interfaces. The transmission. 



87 

however, is exponentially proportional to the absorption coefficient, 
which is why absorbance spectra approximate absorption spectra. The 
two terms will therefore be used interchangeably when describing 
spectra. 

The band gap energy was taken as the point where the slope of the 
absorption curve was a maximum. One unique feature of the Lambda 9 
was that it allowed anywhere from the first to fourth derivative of a 
spectra to be taken. The maximum of the first derivative of the 
absorption edge was taken as the band gap energy. A calculation to 
verify these results will be described in Chapter V. Basically, it 
evolves solving equation A. 2 for a using the absorbance spectra, and 
then plotting a?- verses energy. A direct band gap material will give a 
straight line for this plot, and the extrapolation of the line to = 
gives the optical band gap of the material. 

The temperature dependence of the band gap energy was measured by 
cooling the sample with the cold finger. The sample was mounted in a 
ring sample holder which would then be attached to the end of the cold 
finger. A radiation shield was then positioned around the cold finger 
and a vacuum insulation collar with fused quartz windows was then slid 
over the entire assembly. A vacuum of less than 1 micron pressure was 
then obtained in a few minutes by pximping the system with a Balzer's 70 
1/sec turbomolecular pump. Once this vacuum was obtained in the 
collar, cooling of the sample to 9° K could be accomplished in less 
than 1 hour, although most measurements were made after 2 hours of 
cooling. Any intermediate temperature between 9° K and 250° K could 
achieved with the digital temperature controller and heater assembly. 



88 

The cold finger assembly was mounted on a height-adjustable rack system 
so that once a sample was cooled to low temperatures the cold finger 
assembly could be moved so that measurements on different equipment 
such as the OMA and Raman spectrometers could be accomplished. 

All aspects of the measurements with the Lambda 9 were greatly 
enhanced by interface to the AT computer. Spectra were stored as ASCII 
files which later could be imported into a LOTUS worksheet (Lotus 
Development Co., Cambridge, MA) for manipulation and plotting. The 
BASIC program listed in Appendix A would accumulate spectra in the form 
of a long column of numbers, each number representing the absorption or 
transmission at a given data point. The data interval was based on the 
slit width used for the analysis. The slit defines the bandpass that 
determines the resolution to which features can be resolved. Usually a 
data interval of one- third the slit width was used for the analysis. 
Slit widths from 4 nm down to 0.5 nm were used depending on the 
required resolution. As the slit width is decreased, the signal to 
noise ratio is also decreased. This means that the amount of time that 
is spent at a given data point must be increased. Computer control of 
the spectrophotometer was indispensible for making high resolution runs 
where very slow scan speeds were necessary. 

Photoluminescence and Raman 

Further investigations of the band structure of semiconductors can 
be made by measurement of the emission bands or peaks produced by 
photoluminescence and resonant Raman scattering. Measurement of these 
emissions for this study were made with the two spectrometers described 



89 

above. The majority of the measurements, however, were made with the 
optical multichannel analyzer (OMA) and a 0.33 meter Instruments SA 
monochromater . Most measurements were made at low temperatures, using 
the cold finger to cool the sample. 

The detection system of the OMA spectrometer consisted of a linear 
silicon photodiode array which was 1024 elements wide with the diode 
centers spaced every 25 |im. Approximately 700 of the center diodes 
were intensified to increase their light conversion efficiency. The 
diode array was connected to a multichannel analyzer and the array was 
scanned by the MCA to generate a spectra. Scan rates as fast as 16.6 
msec/scan could be used, although to obtain a higher number of counts, 
scan rates of 500 msec were typically used. The computer system 
controlling the MCA was setup to accximulate a number of scans so that 
signal averaging could be used to reduce noise levels. Also, the dark 
current or background noise could be automatically subtracted from a 
spectrxim. The diode array could be cooled to 5° C with a Peltier 
cooler to further reduce the background noise level. With the 
monochromater and grating combination the MCA window was about 20 nm. 
This is the width of the spectra which was displayed on the screen. 
The grating in the monochrometer was rotated to change the wavelength 
window. 

The third meter monochromater was used with a 2400 lines/mm 
grating. The f-stop or light gathering capability of the monochromater 
was calculated with 



90 



focal length 

f-stop = (4.5) 

grating size 



where grating size is the linear dimension of the grating. With a 55 
mm grating, the f-stop of the monochromater is f6. The reciprocal 
linear dispersion (RLD) of a monochromater is a measure of its ability 
to disperse light, and can be calculated by 

RLD = X (4.6) 

f de 

where d0/dL is the angular dispersion (in rad/mm) and is approximately 
equal to the number of grating lines per mm. The RLD for this 
monochrometer was calculated to be 1.25 nm/mm. The bandpass or 
resolution of a monochromater is usually calculated by multiplying this 
number by the width of the slit used. With a linear diode array 
detection system, however, the resolution is determined by the spacial 
resolution of the array. Assuming that three diodes can resolve a 
peak, which correspond to 75 pm, the resolution of this system was 75 
Mm X 1.25 nm/mm or 0.9 nm. 

A schematic of the experimental setup is shown in Figure 19 and 
photographs of the system are shown in Figure 20a and Figure 20b. An 
f2 lens was used to focus emitted light into the spectrometer and a 100 
mm focal length lens was used to focus the laser beam on the sample. 
Typically a focused spot size of 50 um was used, so with an incident 
power of 1 mW, the focused power density corresponds to 400 W/cm^. As 



91 




Figure 19 Schematic diagram of experimental setup used for measuring 
photoluminescence . 



92 




Figure 20 Photograph of OMA setup which was illustrated in Figure 



93 




b) 

Figure 21 Photographs of setup for Raman spectroscopy, a) shown are 
the computer for acquiring data and the vacuum pumping 
system for the cold finger; b) similar optics setup to OMA 
shovm. Laser beam is brought in from the left, focussed on 
sample in cold finger, and the emitted light is focussed 
into the first slit of the spectrometer, at the far right. 



94 

indicated in the schematic, the laser beam was passed through a 
variable neutral density filter and a sharp cutoff spike filter. The 
variable density filter was used to adjust the power of the incident 
laser beam before being focused on the sample. The spike filter was an 
indispensible item which was used to remove plasma lines from the laser 
beam. Plasma lines originate from the argon plasma that is used to 
generate the laser beam. The lines are incoherent radiation, but they 
are very sharp transitions which can be mistaken as photo Iximines cent 
transitions as they occur over the entire visible wavelength range. A 
given spike filter passes only a very narrow wavelength range of light 
which corresponds to the laser line, therefore, a spike filter can only 
be used with its designated laser line. These filters are rather 
expensive so spike filters for only the 488 nm and 457 nm lines were 
available. The argon laser emits five other lines that occur at 514 
nm, 496 nm, 476 nm, 465 nm, and 454 nm. Plasma lines were removed from 
the other laser lines by using a spatial filter. This filter consists 
simply of a grating and it works by diffracting the laser line at a 
different angle from the plasma lines. The spacial filter only works 
if a high number grating is used or if the laser beam can travel 
several feet before being focused. 

To verify that the OMA was not missing any features of the 
photoluminescence, spectra were measured with the Instruments SA Raman 
spectrometer. This spectrometer utilized a double pass one meter 
monochromater with a photon counting photoraultiplier tube and a digital 
discriminator. Photographs of the experimental setup are shown in 
Figures 21a and 21b. A simple lens system consisting of two piano- 



95 

convex lenses was used to focus the light emitted from the sample into 
the monochromater . Other than this difference, the setup was the same 
as the OMA. With 1800 lines/mm gratings the RLD of the monochromater 
was calculated to be 0.24 nm/mm. The width of the four slits used in 
this instriunent determine the resolution, so with the smallest 
practical slit of 10 um, a resolution of 0.024 nm could be realized. 
The small linear dimension of the gratings (125 mm) with respect to the 
long local length however, reduced the light gathering capability to an 
f8. This turned out to be a very serious limitation for using the 
spectrometer to measure photolviminescence, as integration times of up 
to 5 sec per data point were required. A typical run was made using 
slit widths of 100 um, which gave nearly four times the resolution of 
the OMA, and data was collected every 0.05 nm for 2 sec. For a 20 nm 
window this meant that a single scan would take 15 minutes. At least 
three scans were required to reduce noise levels by signal averaging, 
which meant that for only one wavelength window, 45 minutes were 
required to acquire data. To make the accumulation of data easier, the 
Z-158 computer was interfaced to the spectrometer controller, and is 
shown in Figure 21b. The BASIC program to run the spectrometer was so 
long however, that it had to be compiled to reduce running time. 



CHAPTER V 
RESULTS AND DISCUSSION 



Thin Film Physical Properties 
The primary focus of the experimental results presented in this 
section are on the variation of the microstructure, composition and 
electrical properties of thin films as a function of deposition 
parameters and post deposition treatments. First the properties of as 
deposited thin films will be described and then the variation of 
properties with different heat treatments will be reported. Finally, 
the various physical properties of COSAD thin films will be described. 
The variations presented in this section will be correlated to optical 
properties presented in a subsequent section. 

Microstructure Characterization 
Transmission electron microscopy 

Room temperature depositions. A wide variety of microstructures 
were obtained by altering the substrate temperature during deposition. 
The great majority of thin films made for this study, however, were 
deposited onto substrates held at room temperature. Several advantage 
were realized by using this temperature: l)the deposition process was 
greatly simplified, 2)very high quality thin films could be produced, 
and 3)optimum optical properties were obtained by heat treating films 
deposited at this temperature. These heat treatments, however, 

96 



97 

inevitably increased the grain size of the thin films from values 
obtained with cooled or room temperature substrates. 

The resulting microstructure of thin films deposited at room 
temperature is found to be independent of the substrate material. TEM 
micrographs of three representative thin films are shown in Figures 22, 
and 23. These micrographs show the structure of films 1000 A in 
thickness that were deposited at 1 A/sec onto a carbon support film 
(Figure 22), a silica slide (Figure 23a), and a NaCl substrate (Figure 
23b). All three films display polygonal shaped grains with an average 
grain size of 250 A. Many grains are highly faulted showing a defect 
structure of microtwins and stacking faults. Due to the extremely 
small size of these grains the microtwin orientation cannot be 
determined. Another predominant feature displayed by the 
microstructure of these thin films is the large number of Moire 
fringes, indicated by the highly parallel lines with an average spacing 
of 10 to 15 A. These patterns occur when two crystals overlap each 
other with a specific orientation. When a large number are observed in 
a polycrystalline film it is an indication that the grains of the film 
have a preferred orientation, although the orientation cannot be 
directly determined. 

By performing diffraction experiments, the orientation could be 
determined. The grains were found to be oriented with the basal plane 
of the hexagonal lattice parallel to the substrate plane. This is 
indicated by the strong cubic reflections observed in the selected area 
diffraction pattern (SAD) shown in Figure 2Aa. In comparison to the 
indexed schematic pattern shown in Figure 2Ab, many of the hexagonal 



98 




30 run 

b) 



Figure 22 TEM bright field micrographs of 1000 A thick film deposited 
onto carbon support film, a) low magnification showing 
uniformity of grain size; b) high magnification showing 
microtwins and stacking faults. 



99 




Figure 23 TEM micrographs of thin films deposited onto alternate 
substrates, a) silica glass slide; b) NaCl slab. 





b) 

Figure 24 Selected area diffraction pattern of thin film. a) actual 
pattern; b) indexed schematic pattern. 



101 

reflections appear to be missing; however, upon close examination all 
the spacings for hexagonal wurtzite can be found. The lines for these 
spacings are too faint to show up in the SAD pattern reproduced here. 
The orientation seen here is not dependent upon the substrate since 
similar films were made on both amorphous and crystalline substrates. 
These results are consistent with the early work on thermally 
evaporated CdS.^^ It is not clear why this specific orientation 
results because the close packed plane (i.e. (0002) hexagonal plane) is 
the slow growth plane. This means that the orientation is more closely 
related to the nucleation rate of the thin film rather than the growth 
rate, although this orientation was observed with all substrates 
materials, deposition rates, and at all substrate temperatures. 

The grain size seen here is smaller than the 500 A size reported 
by Martil et al.^^""^^. Grain size determinations by SEM of the films 
studied here do show a different size (see SEM section below) but, 
another reason for the difference can be attributed to differences in 
the sputtering technique used to produce the films studied here. As 
previously described, planar magnetron sputtering results in a lower 
film and substrate heating due to electron bombardment, therefore a 
smaller grain size should result. Higher deposition rates were found 
to increase the substrate temperature. However, even with a deposition 
rate of 5 A/ sec the substrate temperature was found to only increase to 
35° C. Analysis of Figure 25 shows that no significant increase in 
grain size has resulted from the increased deposition rate; however, 
the micrograph does indicate that many more microstructural defects are 
present. The higher supersaturation of the vapor results in additions 



102 





Figure 25 Thin film deposited at high deposition rate, a) TEM 
micrograph showing large number of defects; b) 
corresponding SAD pattern. 



103 

of atoms onto non- favorable positions, which increase the nxamber of 
defects during growth. Higher deposition rates should increase the 
amount of stress induced in the film. Strain determinations by X-ray 
line broadening measurements will be described in a following section. 

Low temperature depositions. Although high quality thin films 
could be produced at room temperature, some films were deposited onto 
substrates cooled to LN2 temperatures to determine if a grain size 
small enough for quantum confinement could be produced. Wolgehmuth et 
al.^^ showed that evaporated material deposited at this temperature was 
found to be amorphous as determined by X-ray diffraction. In that work 
a high supersaturation was used to produce these films which may have 
also contributed to the amorphous structure. A much lower deposition 
rate was used in this study to help promote crystal growth, so that 
combined with the low temperature, a very small grain size would 
result. Figure 26a shows the micrograph of a film deposited at 3 A/ sec 
onto a carbon support film which was attached to a substrate cooled to 
80° K. The "grain" structure that is observed is not due to individual 
grains but rather to a columnar structure with low density areas 
indicated by the low contrast between the columns. The SAD pattern 
shown in Figure 26b indicates that the film is somewhat crystalline, 
but highly faulted as evidenced by the width of the rings. The three 
rings corresponding to the cubic reflections are still present 
indicating that only the nearest neighbor spacing is present. No 
hexagonal lines can be found in SAD patterns of these films. 

An indication of how a film grows on a cooled substrate is shown 
in Figure 27a. This section of a film increases in thickness from left 



lOA 




b) 



Figure 26 Thin film deposited at LN2 temperatures, a) TEM micrograph 
showing low density columnar structure; b) corresponding 
SAD pattern indicating a semi -amorphous structure. 



105 




670 nm 

a) 




30 nm 

b) 



Figure 27 Thin film deposited at LN2 temperatures, a) low 

magnification TEM micrograph showing section of film 
increasing in thickness from left to right; b) high 
magnification TEM micrograph showing island structure. 



106 

to right. The film is supported by a carbon film. At the far left an 
individual island structure is observed. The high magnification 
micrograph shown in Figure 27b indicates that the islands are mostly- 
amorphous, with very small microcrystallites imbedded in the amorphous 
matrix. The microcrystallites are characterized by the darker contrast 
of approximately 80 A in size within the globular structures. The very 
fine mottled structure is the background amorphous structure of the 
carbon support film. The SAD pattern included in the Figure shows that 
only the nearest neighbor spacing is present, and the crystal structure 
is highly distorted. Referring back to Figure 27a, as the film 
thickness increases to the right, the islands grow to form a columnar 
structure. The colxamns become fairly thick before they coalesce to 
form a low density continuous films, indicated at the far left in the 
Figure. 

High temperature depositions. Other attempts to alter the 
microstructure of thin films were made by deposition onto heated 
substrates. Both room temperature and LN2 deposited films reported 
above display large numbers of crystallographic defects, which as 
described in a later section, degrade the optical properties. By 
depositing films onto substrates held at 200° C, microstructures with 
fewer defects could be produced. A higher deposition temperature 
should lead to fewer defects because the deposited atoms have more 
energy to diffuse to more favorable positions. Temperatures above 200° 
C, however, produced films which were very strongly bonded to the 
substrate and hence could not be removed for examination. A 
representative thin film deposited at 200° C which was "f loated-of f " a 



107 




Figure 28 Thin film deposited at high temperature, a) TEM micrograph 
showing large defect free grains; b) corresponding SAD 
pattern. 



108 

silica substrate and its corresponding SAD pattern are shown in Figures 
28a and 28b. The absence of any contrast in many of the grains 
indicates a highly ordered crystal structure. The grains are again 
polygonal in shape with an average size of 800 A. The SAD pattern 
shows that the preferred orientation is still maintained, but several 
of the other hexagonal reflections are now more predominant. Although 
fewer crystallographic defects are observed in these films, the "bulk" 
structure was found to be very nonuniform. Variation in color of the 
film across the substrate indicated a variation in composition and/or 
thickness, which made these films unsuitable for optical studies. Films 
deposited at temperatures greater than 200° C were found to be even 
more nonuniform. The nonunif ormities result from nonuniform thermal 
contact because silica substrates are such poor conductors of heat. 
Thermal contact paste was not used due to concerns about contamination 
from the paste. The nonunif ormities encountered at high deposition 
temperatures will be detailed more thoroughly in a following section on 
the scanning electron microscopy of thin films. 

Heat treated thin films. Once the optical properties of as- 
deposited thin films were determined, it became apparent that to obtain 
optimum optical properties some type of thermal treatment had to be 
conducted on the films to remove the defect structure produced by the 
deposition process. However, due to difficulties with the increased 
bonding strength of thin films to substrates as a result of even mild 
heat treatments, very few TEM micrographs were obtained. The primary 
technique used for observing the microstructure of heat treated thin 
films was by SEM, and the results are presented in the following 



109 

section. The TEM micrographs that were obtained of a heat treated film 
are shown in Figures 29a and 29b, and the SAD pattern for this film is 
shown in Figure 30. A film with an original thickness of 1000 A was 
deposited on a NaCl substrate and then heat treated to 650° C for 4 
minutes. By dissolving the substrate the film was captured on a TEM 
carrier grid. The micrographs in Figure 29 show the film to be very 
porous with an interconnected grain network. This high porosity 
results when a large amount of grain growth occurs in a very thin film 
because there simply is not enough material to produce a continuous 
structure. The grains within the network display an average size of 
2000 A and many dislocations, stacking faults and bend contours are 
indicated by the variation in contrast observed within grains. These 
defects might be intrinsic or they may have occurred when the film was 
"f loated-of f " the substrate. It is interesting to note that very few 
if any microtwins can be observed in this structure. This result 
indicates further that these defects are growth related. The SAD 
pattern shown in Figure 30 displays a single crystal pattern with a 
basal plane zone axis, which means that the preferred orientation 
observed in as-deposited films is maintained after heat treatment, 
although this orientation may have been enhanced somewhat because the 
film was heated on a crystalline substrate. 

Scanning electron microscopy 

As -deposited thin films. The microstructure of thin films was 
determined by SEM for several reasons. As explained above, most heat 
treated thin films could not be removed from their substrates for TEM 



110 




Figure 29 



Thin film deposited on NaCl substrate and subsequently heat 
treated, a) TEM micrograph showing porous network; b) TEM 
micrograph showing dislocations and stacking faults. 



Ill 




Figure 30 Selected area diffraction pattern of thin film shown in 
Figure 29. 



112 

examination. Also, films of useful thickness for optical studies were 
much too thick for TEM. Finally, no specimen preparation was required 
to examine the films, other than mounting the substrate with film on a 
specimen holder- This afforded the ability to examine any type of film 
regardless of thickness or treatment. The limited resolution of the 
SEM, initially, was a problem, especially for as-deposited thin films 
with very small grain size. However, optimization of the technique 
yielded high magnification SEM micrographs (86,000X with 400 A 
resolution) • 

The typical low magnification structure seen in 1.0 \im thick films 
deposited at room temperature is shown in Figure 31a. The micrograph 
shows these films to be of very high quality with very few large 
defects such as pin holes or cracks. A high magnification micrograph 
typical of these films are shown in Figure 31b. By making line 
intercept measurements on Figure 31b, and assximing that the change in 
contrast from point to point is due to a grain boundary, an average 
grain size of 480 A is measured. Although this number is only 
approximate, this size is nearly twice that measured by TEM 
observations and is slightly less than the size observed by Martil et 
al. The difference in size between TEM and SEM observations of the 
films studied here is greater than the difference in magnification 
calibration between the two instruments, therefore the difference is 
most likely due to the fact that the grain size observed at the surface 
of a thin film is not the same as that observed in the interior. Also, 
because the SEM micrographs were obtained near the resolution limit of 
the microscope, there is considerable distortion of the image. The one 



113 




Figure 31 SEM micrographs of as-deposited thin film, a) low 
magnification; b) high magnification showing grain 
structure. 



IIA 

conclusion derived from SEM micrographs that cannot be readily 
determined by TEM relates to the lack of flatness on the surface of 
these films, on this size scale. This may be the result of columnar 
growth on a very fine scale. 

The columnar structure observed by TEM in thin films deposited at 
LN2 temperatures is not readily seen by SEM, as shown in Figure 32. 
The size of the structure observed in this 1.0 Mm film is greater than 
the grain size observed by SEM of room temperature deposited films, 
albeit this is a much lower resolution micrograph so the size may be 
distorted. By referring back to Figure 26 one can see that the width 
of the coluimis is approximately 500 A. The size measured here is 
approximately 800 A, which is similar to the differences between the 
grain size measured by TEM and SEM on room temperature deposited films. 
From Figure 32 it is not possible to determine that a semi -amorphous 
columnar structure is present in this thicker film, which is one 
drawback of using SEM for microstructure determinations. 

Other capabilities of the SEM, however, make the observation of 
other physical properties possible. The nonunif ormities seen in films 
deposited at temperatures greater than 200° C are greatly enhanced by 
imaging backscattered electrons from the film. The energy of 
backscattered electrons is dependent upon composition, so the contrast 
observed in the micrograph of Figure 33 is related to the compositional 
nonunif ormities of a film deposited at 300° C. These results correlate 
well with those reported by Cook and Christy,^'' who found that 
deposition above 200° C yielded poor quality thin films. The large 



115 




Figure 32 SEM micrograph of thin film deposited at LN2 temperatures. 



116 




IMjL^JlMrELJJFMSE 



Figure 33 SEM micrograph of thin film deposited at 300° C, 
backscattered electron image. 



117 

differences in sticking coefficients of cadmixim and sulfur at high 
temperatures is responsible for this result. 

Heat treated thin films. Furnace heat treatments similar to those 
carried out by Martil et al.^^ were used to study the grain growth of 
thin films, in addition to heat treatments by rapid thermal annealing. 
Optimum film properties were obtained by furnace heat treating to 500° 
C for 5 hours, or to 650° C for 4 minutes. Typical microstructures 
obtained of 8000 A thick films subjected to these heat treatments are 
shown in Figures 34a and 34b. The grains appear isotropic with an 
average size (determined by a line intercept method) of 2850 A for the 
500° C treatment and 3000 A for the 650° C treatment. These grain 
sizes are similar to the size observed in the porous network displayed 
in the TEM micrograph of a very thin film heated to these temperatures 
(Figure 28). Thermal shock, mismatch of the film and substrate 
expansion coefficients, or excessive grain growth could be responsible 
for the cracking of films which is sometimes observed when films are 
subjected to fast furnace heating rates. This cracking is shown in 
Figure 35. 

From other heat treatments at 200 and 350° C for times of 5 hours, 
it was found that the grain size exhibited two regions of growth. This 
is shown in Figure 36, which is a plot of average grain size versus 
reciprocal temperature, for constant time. The two regions of growth 
exhibited in the plot are a classical indication that two different 
mechanisms are responsible for grain growth in these films. At low 
temperatures the grain growth is slow with a nearly constant rate. 
This is an indication that grain growth occurs only by grain boundary 



118 




Figure 3A SEM micrographs of heat treated thin films, a) heat 

treated 650° for A min; b) heat treated 500° C for five 
hours . 



119 




Figure 35 Low magnification SEM micrograph of thin film shown in 
Figure 3Aa. 



120 




Figure 36 Plot of average grain size versus heat treatment 
temperature. 



121 

diffusion, which is a low energy process. At higher temperatures, 
enhanced diffusion of impurities at the grain boundaries results in a 
large increase in grain growth. Grain growth is therefore controlled 
by the activation energy for impurity diffusion. The curve appears to 
flatten out at the highest temperatures, which is possibly due to two 
effects. First, since the film is of a finite thickness, there must be 
a limit on the maximum size of the grains. Generally the largest grain 
size obtainable should correspond to the thickness of the film, but 
when a film is in contact with a substrate, this constraint will depend 
upon the interface energy between the film and substrate. The more 
likely effect, however, is due to a loss of material at higher 
temperatures. Thin films were found to totally evaporate at 800° C in 
a very short time, even though the sublimation temperature of bulk CdS 
is 925° C. At 650° C for long times a loss of material was also found 
to occur which would disrupt the grain growth process. 

Although some of the optical properties were optimized by heat 
treatments to 650° C, the larger grains and cracking of these films 
increased the amount of light scattering. Rapid thermal annealing 
(RTA) permitted high temperature treatments for very short times, which 
resulted in very little grain growth and no cracking of the film. 
Figure 37 shows a high magnification micrograph of a film that received 
an RTA treatment at 650° C for 30 sec. The film displays an average 
grain size of only 900 A, which is considerably smaller than the size 
obtained by furnace heat treatments to this temperature. Since this 
film was subjected to a very large thermal stress, it appears that the 



122 




Figure 37 SEM micrograph of thin film subject to rapid thermal 
anneal (RTA) heat treatment. 



123 

cracking seen in the furnace heat treated films is a result of the 
large and rapid grain growth that occurs. 

X-ray diffraction experiments 

Diffraction spectra. Although SEM observations gave an indication 
of microstructure morphology of thicker films, the technique did not 
provide any crystal structure information. To determine the 
crystallography of thicker films X-ray diffraction (XRD) was used. The 
computer controlled dif f ractometer used for these measurements greatly 
facilitated the determination of microstructure changes with heat 
treatment, as well as permitting quantitative analysis of X-ray 
spectra. Programs for calculating diffraction peak line broadening due 
to strain and size effects and for calculating precision lattice 
constants were available with this system. 

The diffraction spectra obtained from films 8000 A thick, 
deposited at three different temperatures are displayed in Figure 38 
and the diffraction spectra obtained after heat treating these films 
are shown in Figure 39. Many of the features observed in SAD patterns 
are evident in these X-ray diffraction patterns. The broad band peak 
centered at = 20° in all patterns is due to amorphous scattering from 
the substrate. 

In Figure 38a, of a film deposited at LN2 temperatures, the 
amorphous peak is much more intense and the intensity of the (0002) 
peak is considerably reduced. This supports the results obtained from 
TEM analysis that the structure of these films is highly disordered, 
but there is some indication of crystallinity . After heat treatment. 



12A 




Figure 38 



X-ray diffraction patterns of as-deposited thin films, a) 
deposited at LNo temperatures; b) deposited at room 
temperature; c) deposited at 300° C. 



125 

Figure 39a shows the amorphous peak to be greatly reduced and 
crystallization has taken place with the strong cubic preferred 
orientation. 

Figure 38b shows that films deposited at room temperature exhibit 
a strong cubic preferred orientation which is shown by diffraction at 
(0001) where 1=2,4,6. The other hexagonal lines seen in SAD patterns 
are not as readily observable in this pattern, particularly the other 
two lines which are closely spaced to the (0002) peak. Those two lines 
were clearly resolved in SAD patterns, but appear to be totally absent 
here. This shows how much more sensitive electron diffraction is to 
the local crystal structure. When this film is heat treated, the 
preferred orientation is maintained as shown in Figure 39b. All 
diffraction peaks become much narrower and more intense, as an 
indication of grain growth. Figure 39c indicates that when this film 
is heat treated to an even higher temperature, the orientation is 
maintained, but the intensity of other hexagonal diffraction lines is 
increased. 

The most predominant hexagonal lines are seen in the XRD pattern 
obtained from a film deposited at 300° C, shown in Figure 38c. Again a 
preferred orientation is indicated, but other hexagonal lines are shown 
to occur in a different ratio to the cubic lines, which indicates there 
is another orientation present. This second orientation was not 
observed in the TEM analysis because the film examined was deposited at 
200° C. It appears this orientation only occurs at higher 
temperatures . 



126 



2 











(0004) 


1(0002) 


|(000«) 






H (1l24) 

.J^Jl^ 









-| T 1 1 1 1 n 

M 7S 66 »• 4* M 2* 



a) 



6O0 CPS FUU.»CAL£ 







I ' I I I I I p 

M 7» •6 M 4» M 2* 





) 




too cr» 








> 


FULLSCALI 








m 


1 









TT I ^ 1 1 1 r- 

7t as 65 46 a« 26 



DEGREES TWO THETA 

c) 



Figure 39 X-ray diffraction patterns of heat treated thin films from 
Figure 38; a) film of Figure 38a, heat treated 500° C, one 
hour; b) film of Figure 38b, same heat treatment; c) film 
of Figure 38b, heat treated 720° C, ten minutes. 



127 

Strain determinations. Residual strain in thin films can be 
manifested as both a shift and a broadening of X-ray diffraction lines. 
However, broadening of X-ray lines due to strain will only occur if a 
nonuniform microstrain is present (usually due to plastic deformation). 
Broadening usually is a result of a very small grain size. Digital 
manipulation of the diffraction data permitted determination of peak 
broadening due to nonuniform strain and grain size effects by use of an 
utility program available in the APD computer. The computer program, 
however, does not calculate the uniform strain which would produce a 
shift in peak positions. Residual uniform strain was determined by 
comparing the position of the (0006) diffraction line in thin films to 
the position observed in a powdered sample of CdS. By differentiating 
Bragg 's law, the change in lattice parameter due to a uniform strain 
was calculated from the shift observed in the peak positions. 
Figure 40a shows a comparison of the broadening and peak position seen 
in films deposited at two different rates and the line width and 
position which results when these films are fully annealed. The tick 
mark shows the position of the (0006) line of the powdered sample. The 
calculated uniform strain which would produce the observed line shifts 
and the effective mean nonuniform strain and effective mean grain size 
that would produce the broadening are listed in Table 2. Also shown 
are the values which would cause the broadening if only grain size 
effects were present. 

Thin films were considered to be uniformly strained, therefore the 
results calculated by the APD program for nonuniform strain are not 
considered relevant. The grain size effects listed in the last column 



128 




■ I I 1 r 1 1 1 1 T 1 I 1 I 1 I I 

as.a 83.fl »4.« 83.0 W.0 e/.t ae.a 89.« m.« 



8.72 
6.719 - 
6.718 - 
8.717 - 
6.716 - 
6.715 - 
6.714^ - 
6.713 - 
6.712 - 
8.711 - 

8.71 
6.709 
6.708 
6.707 
6.706 
8.703 - 
6.704. 
6,703 - 
6.702 
6.701 H 
6.7 



— I — 
0.2 



0.4 



0.8 



— I — 
0.8 



— 1 — 
1.2 



1.4 



— 1 — 

i.e 



I 

1.8 



H-R FUNCTION 



b) 

Figure 40 Analysis of X-ray diffraction spectra. a) (0006) X-ray 

diffraction peak of two as-deposited and one furnace heat 
treated thin films showing position shift and broadening 
due to strain and grain size effects; b) plot of c-axis 
lattice parameter versus Nelson-Riley function for 
precision lattice determination. 



129 



TABLE 2 



Calculated strain and size effects which would produce the 
X-ray line shifts and broadening observed in diffraction patterns 



Sample 


Effective 
Nonuniform 
Mean Strain^ 
% 


Effective 
Mean Size-'- 
A 


Calculated 
Uniform 
Strain^ 
% 


Size 
Effects 
Only^ 
A 


ASDP 1 A/sec 


0.189 


HAS. 7 


0.16 


252.7 


ASDP 5 A/ sec 


0.179 


1983.6 


0.39 


28A.3 


FHT 650 C 


O.OAl 


2528.0 


0.08 


1005.2 


RTA 650 C 


0.122 


837.8 


0.12 


332.6 



Key: ASDP - as-deposited; FHT - furnace heat treated; 
RTA - rapid thermal anneal. Heat treated films were deposited at 

1 A/sec 

Calculated with utility program available in APD computer. Mean 
values are given which would combine to cause the observed broadening. 

■Calculated from shift in X-ray line position. 

'Calculated with utility program available in APD computer. Values of 
grain size which would produce the observed broadening. 



130 

of the Table do, however, correlate closely to grain size observed by 
TEM for as-deposited thin films. The shift of the higher rate film to 
a smaller two-theta value indicates the presence of a tensile stress. 
Higher stress levels in higher rate films may be due to growth related 
defects and the inclusion of impurities. The higher stress level in 
these films causes the film to exfoliate within several hours after 
exposure to the atmosphere. Figure Ala shows an optical micrograph of 
a film exhibiting the onset of this exfoliation, and Figure Alb 
displays the appearance of a film after several days of exposure. The 
appearance of the exfoliation shown in Figure Ala is characteristic of 
a tensile stress. Exposure to atmosphere leads to the absorption of 
water on the film surface, which apparently increases the stress to a 
value large enough to cause the film to break away from the substrate. 
Some type of stress related corrosion process may also contribute to 
this result. Lower deposition rate films were found to be stable with 
exposure to the atmosphere indefinitely, so X-ray line shift due to 
uniform strain was expected to be less. 

Precision lattice constant. A calculation of the precision 
lattice constant of heat treated thin films was made from the positions 
of diffraction peaks measured from thin films. Due to the strong 
preferred orientation only the c-axis lattice constant could be 
determined and because small two-theta values had to be used, the 
calculated lattice parameter was plotted versus the Nelson-Riley 
function. ^-^ The plot shown in Figure AOb indicates an extrapolated 
lattice parameter of Cq=6.707 A, which is 0.09% less than the JCPDS 
card file value of 6.713 A.^-^ 



131 




Figure Al Optical micrographs of exfoliated thin film, lOOX 

magnification, a) after four hours exposure to atmosphere; 
b) after two days exposure. 



132 

Compositional Analysis 

Stoichiometry determination of target material 

The first parameter needed to establish the composition 
stoichiometry of thin films was the composition of the target material 
from which films were made. Once this composition was determined, the 
target material could be used as a standard for all other analytical 
techniques. Both induction coupled plasma (ICP) and electron probe 
microanalysis (EPMA) were used to determine the stoichiometry of the 
target material. These two techniques are by far the most quantitative 
because the results are determined by comparison to known standards. 
Additional measurements were made by standardless techniques which rely 
on sensitivity factors for quantitative analysis, such as: X-ray 
photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). 
The manufacturer of the target material claimed the composition to be 
slightly sulfur rich, but as can be seen Table 3, both ICP and EPMA 
data indicate the target material to be slightly cadmiiam rich, although 
the variation is within a few standard deviations of each other. In 
either case the results indicate the target material to be 
stoichiometric to 0.50 %, which for the other analytical techniques 
used to measure thin films, is an acceptable variation. XPS gives a 
result that is very close to the excepted value; however, AES shows a 
very large difference from the other results which demonstrates that 
the technique is not useful for determining absolute concentrations, 
but as described below, the technique is very sensitive for determining 
differences between samples. 



133 



TABLE 3 



Concentrations in atomic percent of cadmium and sulfur 
found in target material 



Analytical 
T echnique 



Cd concentration 



S concentration 



ICP 
EPMA 
XPS 
AES 



50.96 +/- 0.09 

50.33 +/- 0.21 
51 

60.7 



49.04 +/- 0.12 
49.67 +/- 0.21 
49 

39.3 



134 

The results for the four different analytical techniques used for 
determining the composition of thin films are listed in Table A. The 
absolute values of the sulfur and cadmixim concentrations are shown to 
vary greatly between different techniques; however, the difference 
between as-deposited and heat treated thin films is nearly the same in 
each case, except for the XPS results. Films that were heat treated to 
650° C were used because of their use for optical properties studies. 
They were also found to show the greatest difference. The discrepancy 
in the XPS results for heat treated thin films and the implications of 
the results of the other techniques will be discussed below. The 
primary indication of these results, however, is that as-deposited thin 
films contain ~ 2% excess sulfur and once they are heat treated, this 
excess increases to ~ 4%. As indicated by the precision lattice 
parameter described above, the unit cell of heat treated thin films is 
very close to the ideal structure of CdS, so very little of this excess 
sulfur is interstitial, or very few cadmixim vacancies are present. 
This means that the sulfur must be a second phase. 

Analytical techniques 

Use of electron probe microanalysis (EPMA) for determining the 
composition of thin films was first thought to be the best method and 
most quantitative, for the same reasons stated above. Not even 
qualitative results, however, could be obtained. The standard ZAP or 
<))pz^^ programs used to calculate composition consistently gave results 
of less than 100 weight percent for the composition of thin films. 
Even a thin film program which relies on some approximations, did not 



135 



TABLE 4 



Comparison of results from different analytical techniques 
used to determine the composition of thin films 



As Deposited 



Heat Treated 



Analytical 


Atomic % 




Atomic % 


Technique 


Cd 


S 




Cd 


S 


EDS 


47.7 


52.3 


46 


.8 


53.2 


XRF 


44.6 


55.4 


43 


.1 


56.9 


XPS 


50.6 


49.4 


70 


.9 


29.1 


AES 


37.3(47.6) 


62.7(52.4) 


35 


.7(46.0) 


64.3(5^ 



136 

produce a reasonable result. It could not be determined if there was a 
problem with the software or the instrument. Even if EPMA results 
could have been obtained it would have been impractical to analyze 
every film produced and every heat treatment studied. 

X-ray fluorescence was originally considered to be the best 
technique for the general analysis of thin film composition for several 
reasons: l)the intensities of the X-ray fluorescence lines should be 
directly proportional to the amount of material present in the thin 
film, 2)high peak intensities gave a large signal to noise ratio, 3)no 
sample preparation was necessary and analysis was quick (typically 15 
minutes), 4)film thickness could be readily determined and 5)the 
results were found to be very reproducible. Different films that were 
deposited iinder exactly in the same conditions and heat treated exactly 
the same way were found to give nearly the same peak intensities for 
the cadmium and sulfur lines. An example of this reproducibility is 
shown in Figure 42 which shows peak intensity and integrated peak 
intensity ratios for several identical samples. A problem with the 
technique occurs if a linear correlation factor is used to calibrate 
the XRF spectra obtained from thin films: the amount of sulfur appears 
to be too high, when compared to the other techniques. The correlation 
factor is based on the ratio of the intensities of the cadmiiim and 
sulfur lines obtained from the target material, assuming a 
stoichiometric composition for the target. One possible explanation 
for the high value is that a matrix effect that alters the ratio of the 
X-ray lines measured for the target material may not occur in a thin 



137 



0.3 -r 

0.28 
0.26 



3. 



0.24 H 
0.22 

0.2 
0.18 A 
0.18 
0.14 -j. 
0.12 - 

0.1 - 
0.08 
0.06 
0.04 - 
0.02 



\ 



V 



X 



/ 



\ 



\ 



I 



\ 



I 



7^ 



i 



2^ 



F1-a.7 F2-8.4 F3-9.3 F4-9.4 F9-8.9 F6-8.0 F7-7.a FB-10 
_ FILM NUM BER 

C^Zl PEAK INTENSITY fV^ INTEGRATHJ AREA 



F9-8.0 F1 0-8.0 



Figure 42 X-ray fluorescence sulfur to cadmium peak ratios for 
several identical thin films. 



138 

film, which would lead to an incorrect correlation factor and incorrect 
estimation of the composition of thin films. 

The reproducibility obtained by XRF could not be accomplished by 
energy dispersive spectroscopy (EDS) obtained during SEM observations. 
This was primarily due to the inability to measure the specimen current 
of the electron beam used to generate secondary electrons for imaging 
and X-rays for composition analysis. Widely different intensity values 
were often obtained from the same sample when measured at different 
times. This problem was alleviated to some degree by mounting a piece 
of target material on each sample mount so that the intensities of the 
X-ray lines obtained from thin films could be normalized to a standard. 
A hole was drilled in the sample mounts so that the surface of the 
target material would be level with the substrates and geometric 
differences would be reduced. Again, a correlation factor was used to 
calculate the composition of thin films based on the intensities of the 
X-ray lines measured from the target material. The matrix effect that 
might occur in XRF would be less pronounced in EDS, because the 
electron penetration depth that generates X-rays is much less than the 
X-ray line that produces the secondary fluorescence in XRF. 

The absolute values of the composition obtained by Auger electron 
spectroscopy (AES) are considered to be reasonable since the accuracy 
of the technique is limited to +/-30% when published values of the 
sensitivity factors are used.^^ However, since very similar values for 
the composition of the target material were obtained, a very simple 
correction factor can be applied to the sensitivity factors to give the 
correct results. The corrected concentrations are given in parenthesis 



139 

in Table 4, and they show comparable results to the other techniques. 
Typical AES spectra obtained from target material and heat treated thin 
films are shown in Figure 43a and 43b. The shape of the peaks are 
qualitatively the same, although the heat treated spectrum has been 
shifted to higher energy due to a charging effect. The difference 
between the ratios of the Cd peak-to-peak distance to the sulfur peak- 
to-peak distance gives a quantitative indication of the difference in 
the concentration of cadmium in the two samples. AES should be very 
quantitative for measuring the differences between similar samples, so 
the values obtained most likely represent the true difference, although 
this technique does not indicate how the sulfur exists in the 
structure. 

To attempt to determine the chemical state of the sulfur. X-ray 
photoelectron spectroscopy (XPS) was used. This technique is very 
sensitive to changes in the binding energy of core electrons, which 
reflects changes in the local chemical environment.^^ The presence of 
significant concentrations of free sulfur, would either shift the 
sulfur lines, split, or broaden them. A high resolution scan of the 
sulfur 2p transition for an as-deposited (ASDP) and a furnace heat 
treated (FHT) thin film are shown in Figure 44. Both films were 
deposited at room temperature and at a rate of 1 A/sec. Very little 
shift is observed and no significant broadening is seen. The position 
of the 2p transition for free sulfur would be at a binding energy of 
165 eV. The only unique feature is the two shake-up peaks, labeled SU 
and both of these show very little shift in energy. The satellite peak 
labeled SAT is an artifact peak and has no meaning. 



140 



18 

9 




8 I 


5 


V - 




^6 : 

ft - 




biS 













"I — I — \ — I — \ — \ — I — I — I — I — I — I — I — I — I — r 
).e 158.8 288.8 258.8 386.8 358.8 488.8 458.8 588.8 558.8 

KINEHI^ EHER6Y, ell 



3.8 



18 
9 

8 



0) 



u 5 

i 

'^4 
u 

I'' 



s 






— 1 1 r 


^ — 1 1 \ 1 r 1 1 1- — -r 


ll 



188.8 158.9 288.9 259.8 388.9 359.8 489.9 459.9 589.9 ' 559.9 698.8 

KINETIC Em, eU 
b) 

Figure 43 Auger electron spectroscopy survey scans, a) Target 
material; b) heat treated thin film. 



141 



CTS 




, 1 1 ■ ■ I ■ •——> ■ rr — ■ ■ • ■ I 

188 170 ^160 150 

BE 



Figure 44 X-ray photoelectron spectroscopy (XPS) high resolution scan 
of sulfur 2p peak, exhibited by an as-deposited (ASDP) and 
a furnace heat treated (FHT) thin film. Heat treated thin 
film peak shifted slightly to higher binding energy. 
Shake-up peaks are indicated by SU and satellite peak is 
marked SAT. 



142 

As indicated in Table 4, there is significant differences between 
as-deposited and heat treated thin films. The concentration 
calculations in XPS are based on quantification factors and on 
integrated peak intensities. Since the factors are constants, the 
result presented in Table 4 indicate that the integrated area of the 
cadmium peak has nearly doubled in heat treated films as compared to as 
deposited. A high resolution scan of the cadmium 3p5 and 3p3 
transitions is shown in Figure 45. Surprisingly there is a significant 
shift in the peak position in addition to a broadening. By comparison 
to published XPS spectra on cadmium compounds^^ this shift could 
possibly correspond to cadmium bonded to oxygen. The survey scans 
displayed in Figure 46 and Figure 47 exhibit oxygen Is peaks in both 
films and the peak appears to be more intense in the heat treated film 
(compare the height of the Is peak to the Cd 3p3 peak). Figure 48a 
displays a high resolution scan of the oxygen peak and Figure 48b shows 
the same scan after a one minute sputter etch, which removes 
approximately 100 A. The large reduction in the intensity of this peak 
after sputtering is an indication that a thin oxidized surface layer is 
present. Unfortunately, a high resolution scan of the Cd 3d peaks was 
not acquired after the sputter etch to determine if the shift in the 
peak position is due to the oxidized surface layer. Analysis was 
performed on a second XPS instrument (Perkin-Elmer) which was equipped 
with a sputter gun. The results for a heat treated film before and 
after a one minute sputter etch are shown in Figure 49. Several 
differences between this spectr\im and the spectrum obtained with the 
Kratos XPS can be observed. First, the 3d peaks in Figure 49 are 



143 



CTS 




Figure 45 



XPS high resolution scan of cadmium 3d3 and 3d5 peaks of an 
as-deposited (ASDP) and furnace heat treated (FHT) thin 
film. 



144 



CTS 




Cd3d5 




Cd3d3 










0000. 
2000. 


Cd3p3 J\ 
□ Is >/ 

Cd3pll 


Cls S2p 




0. 




1 



leoe see eeo ^ee zee e 



Figure 46 XPS survey scan of as-deposited thin film. 



145 





Cd3d3 


CdSdS 


see. 

CTS 






200. 










Cd4d 


100. 




Cts S2p 
1 S2m 





'~ ' 1 ■ • • » • 


1 1 1 I 1 — 1 — — — ■ • 1 



— I ■ • ■ 1 ■ 1 1- 

600 400 200 

BE 



Figure 47 XPS survey scan of furnace heat treated thin film. 



146 




Figure 48 XPS high resolution scans of oxygen Is peak, a) furnace 
heat treated thin film; b) after one minute sputter etch. 



147 




Figure 49 XPS high resolution scan of cadmium 3d3 and 3d5 peaks 

obtained from Perkin-Elmer instrument. Spectrum are of a 
heat treated thin film, as-received and after a one minute 
sputter etch. 



148 

nearly twice as wide as the peaks seen in Figure 45. Second, the peaks 
are shifted to lower binding energy and third, a very definite doublet 
peak is observed. The first two differences may be due to charging 
effects. Charging was not observed in the Kratos spectrum (Figure 45) 
because a low energy electron flood gun was used to compensate the 
build-up of static charge. The clear resolution of a doublet that 
was not resolved in Figure 45 is, however, an anomalous result. Also, 
unexpectedly, the Cd peaks have shifted in the opposite direction after 
the sputter etch. This may be due to preferential sputtering of the 
sulfur, so that more metallic cadmium is present. It may also be due 
to a change in grounding potential, which would effect the static 
charge built-up on the sample. 

Apparently XPS is not sensitive enough to determine if the small 
amount of excess sulfur present in thin films exists as a distributed 
second phase. The final technique that was used to determine this 
distribution is X-ray mapping. Although this type of X-ray analysis by 
EPMA would have been the most quantitative, the probe size used in the 
EPMA is not small enough to spatially resolve the grain boundaries of 
thin films, which is where the sulfur is most likely to exist. By 
using EDS X-ray mapping with the SEM, high spatial resolution could be 
obtained. Simultaneous maps of sulfur and cadmixam were acquired at a 
magnification of 20,000X, which represented about 150 grains. A low 
energy electron beam of lOKV was used to reduce to volume of material 
sampled and a high density 256 x 256 image, with 8 bit resolution, and 
a 200 msec dwell time was used to generate the X-ray maps. Even at 
this high magnification, if the cadmium and sulfur were evenly 



1A9 

distributed, an image with very little contrast should result. The 
area sampled is displayed in Figure 50, which is a digitized SEM image. 
The contrast that is observed is due to the grain boundaries of the 
surface grains and can be compared to the contrast observed in a normal 
SEM micrograph, shown in Figure 39. Figure 51a shows that the cadmium 
map has a low even contrast, but the sulfur map shown in Figure 51b is 
highly mottled, indicating an uneven distribution of sulfur. This 
indicates that the sulfur is distributed as a second phase, at the 
grain boundaries. 

Co-Sputter Alternating Deposition 
Microstructure 

The purpose of making COSAD films was to produce a two phase 
structure, in which very small crystallites of CdS are embedded in a 
glass matrix. The intent of this phase of the research was to 
determine if a structure could be produced in a thin film with small 
semiconductor microcrystallites isolated in a dielectric matrix with 
the crystals small enough for quantum confinement. Thin films made by 
COSAD were therefore primarily analyzed by transmission electron 
microscopy, since the required size of the crystallites was less than 
100 A. By changing the ratio of deposited CdS to glass different 
structures could be produced. The most stable films were those made 
with a small ratio of CdS to glass, while films made with equal ratios 
were found to phase separate during examination in the TEM. An example 
of the two extremes of the variation that could be produced will be 
described here. 



150 



QRTEC inm PRQCESSIHS 




Figure 50 Digitized image from scanning electron microscope, 
secondary electron image. 




Figure 51 Digitized element maps. a) high magnification cadmium 
X-ray map; b) high magnification sulfur X-ray map. 



152 

COSAD films with equal proportions of semiconductor and glass were 
the first to be produced. These films will be referred to as COSADl. 
A representative film which was deposited onto a carbon support film is 
shown in Figure 52a. The structure appears very indistinct and appears 
to be amorphous, but the SAD pattern shown in Figure 52b indicates 
there is some crystallinity in the structure. If the electron beam is 
condensed on this type of film, the structure is found to phase 
separate very quickly, as displayed in Figure 53a. The SAD pattern in 
53b shows that a significant amount of crystallization has occurred and 
the pattern indicates a hexagonal CdS non-oriented crystal structure. 

If this type of film is furnace heat treated a significant change 
in microstructure also occurs. Heat treating to 500° C results in 
total phase separation and the growth of spherical shaped CdS 
crystallites, with an average size of 300 A. This structure is shown 
in Figure 54. By heating this type of film to 650° C, these 
crystallites are found to grow and form a fairly evenly dispersed 
bimodal size distribution microstructure, which is shown in Figure 55a. 
The average size of the larger spherical crystals is 700 A and the 
smaller crystallites have an average size of 200 A. The SAD pattern 
for this structure is shown in Figure 55b. All the reflections for 
hexagonal CdS are present, in addition to some reflections for an 
unidentified phase. Because these reflections are so closely spaced in 
the pattern, it was not possible to determine their source by dark 
field imaging. A typical dark field image obtained from the inner most 
diffraction rings is shown in Figure 56. The bright crystals in the 
image are those which are properly oriented for diffraction and as can 



153 




50 nm 

a) 




Figure 52 TEM micrograph of COSADl thin film, a) as-deposited on 
carbon support film; b) corresponding SAD pattern. 



15A 




155 




100 nm 



Figure 54 TEM micrograph of COSADl thin film, after heat treatment 
500° C for one hour. 



156 




b) 



Figure 55 TEM micrograph of COSADl thin film, a) after heat 
treatment to 650° C for 30 min.; b) SAD pattern. 



157 




200 nm 



gure 56 Dark field TEM image of COSADl thin film shown in Figure 
55a. 



158 

be seen, crystals of both size distribution are lit up. The dark field 
image also shows that several of the large crystals are twinned, 
indicated by the change in contrast within a crystal. Another 
abnormality exhibited by the higher temperature heat treated films was 
the additional appearance of large faceted shaped crystals, shown in 
Figure 57. The crystallographic identity of these crystals could not 
be determined; however, they were only found in certain areas of the 
film, and they may correspond to some type of contamination. 

COSAD films which were made by controlling the alternating 
deposition process with a computer controlled stepping motor could be 
produced with very low ratios of CdS in the glass. Alternating layers 
of material could be produced by stopping the substrate over a source 
for a given amount of time. This permitted the deposition of several 
monolayers in each pass. Ratios which resulted in about 5% CdS 
dispersed in the glass (designated C0SAD2) resulted in structures such 
as the one displayed in Figure 58a. The extremely fine dark structure 
in the micrograph is most likely due to CdS microcrystallites ; however, 
the accompanying SAD pattern (Figure 58b) indicates the structure to be 
amorphous. When the electron beam is condensed on this type of film a 
phase separation is shown to occur, but as exhibited in Figure 59a it 
grows to a smaller size than shown in Figure 53a. The accompanying SAD 
pattern in Figure 59b also indicates that the structure is still 
amorphous . 

When this type of film is heat treated to 650° C, two different 
structures result. First, large semi-faceted shaped crystals are 
exhibited as shown in Figure 60a. Although a single crystal 



159 




200 run 



Figure 57 TEM micrograph of heat treated COSADl thin film, showing 
large unidentified crystals. 



160 




100 nm 



a) 




Figure 58 TEM micrograph of C0SAD2 thin film. a) as-deposited; b) 
SAD pattern indicating amorphous structure. 



161 





Figure 59 TEM micrograph of C0SAD2 thin film, a) after ten minute 
exposure to condensed electron beam; b) SAD pattern. 



162 




200 nm 




b) 



Figure 60 TEM micrograph of C0SAD2 thin film, a) after heat 

treatment to 650° C for 30 min, showing large unidentified 
crystals; b) SAD pattern showing both single crystal 
pattern from large crystals and diffuse pattern from thin 
film. 



163 

diffraction pattern could be obtained from these crystals (shown in 
Figure 60b), they could not be identified. Again these large crystals 
were foxind to be unevenly distributed on the film and therefore 
considered to be due to contamination. At very high magnification, the 
second structure of these films can be observed. Very small spherical 
microcrystallites , of 50 A approximate size are shown in Figure 61. 
Only a very diffuse diffraction pattern shown in Figure 60b could be 
obtained, so it is difficult to identify the microcrystallites as CdS, 
although the pattern does resemble the pattern of the semi -amorphous 
LN2 film described above. A large number of these microcrystallites is 
seen, so a stronger pattern should result. This particular film, 
however, is relatively thick (= 3000 A), so it only appears there are 
many microcrystallites. The pattern is diffuse because the glass 
matrix has a much larger volume fraction compared to the 
microcrystallites . 

Composition 

Very little compositional analysis was carried out on COSAD films, 
primarily because most of the films made were for TEM analysis, and 
therefore were too thin for quantitative analysis. The composition of 
a few of the phase separated particles discussed above however, were 
determined by EDS while imaging the film in the STEM mode of the JEOL 
200 CX electron microscope. The particles were found to contain 
cadmium and sulfur, but the intensity levels for these two peaks were 
too low to get an accurate determination of the stoichioraetry. This 
was particulary true of the particles displayed in Figure 61. A 



164 




50 nra 



gh magnification TEM micrograph of heat treated C0SAD2 
in film showing fine dispersion of microcrystallites . 



165 

typical EDS X-ray spectrum obtained while operating the JEOL 200 CX in 
the STEM mode is shown in Figure 62. This spectrum was acquired from 
the film displayed in Figure 61 and it shows that the particles are 
CdS. Except for silicon, the other constituents of the glass matrix 
(e.g. Ca and K) could not be determined with this technique because of 
their low concentration and the thickness of the film. The copper peak 
originates from the TEM support grid. 

The compositions of thicker films made for optical analysis were 
determined by XRF. One difficulty encountered in using this technique 
was that the potassium Ka line overlapped the cadmium L3 line, so the 
amount of potassivim could only be roughly determined. For COSAD films 
deposited on silica substrates the total amount of silicon also could 
not be determined, due to fluorescence of the substrate. The relative 
amounts of cadmium and sulfur, however, could be determined and it was 
found that peak intensity ratios very close to those of as deposited 
pure CdS films were obtained with COSAD films with both high and low 
amounts of CdS. A few COSAD films were deposited on single crystal 
sapphire to determine the composition separately from that of the 
substrate. 

Thin Film Optical Properties 
Now that the physical properties of thin films have been fully 
described, the results that are central to this dissertation will be 
presented in this section. These results describe how the changes in 
microstructure produced by various treatments affect the optical 
properties. UV-visible absorption, photoluminescence, and resonant 



166 




Figure 62 EDS X-ray spectrum of COSADl film. 



167 

Raman scattering were utilized at both room temperature and low 
temperatures to fully characterize the optical properties of thin 
films . 

Absorption Spectra 

UV-VIS absorption was used in this study to establish the 
relationship between thin film processing conditions and their effect 
on the band structure of the material. The main emphasis of this study 
as stated before, was to produce a thin film with the same optical 
properties as those of bulk single crystal material. From the 
measurement of absorption spectra, it was possible to determine the 
presence of band defects such as band tailing, to determine the 
temperature dependence of the band gap energy and at low temperatures, 
to observe exciton transitions. 

By examining the shape of the absorption edge, a qualitative 
measure of the band gap structure could be determined. At energies 
below the absorption edge, the sharpness of the transition from low 
absorption to the edge gives an indication of the amount of band 
tailing. Only transitions which conserve momentum are allowed, 
therefore, for a direct band gap semiconductor, only k equals zero 
excitations occur without phonon scattering. When a great many 
crystallographic defects are present in a material, then the band 
structure will not be well defined. The presence of defects permits 
transitions at other values of k to be quantum mechanically possible, 
which in the absorption spectra is manifested as a tailing of the 
absorption edge at low energies. 



168 

Room temperature UV-VIS absorption 

The position of the room temperature band gap absorption edge that 
was measured for as-deposited thin films was found to vary slightly 
with film thickness and deposition rate, but mostly with substrate 
deposition temperature. Films ranged in color from straw yellow to 
dark orange to nearly black, as the deposition temperature was varied 
from 300° C to LN2. Typical spectra of films deposited at 300° C, room 
temperature, and LN2 using an absorbance scale are shown in Figure 63. 
The variation in absorbance spectra with film thickness is shown in 
Figure 64. As previously described, absorbance spectra show features 
above the band gap with more detail than transmission spectra. By 
taking the maximum slope of the absorbance, an approximate value of the 
band gap absorption edge is given and as can be seen in Figure 63, this 
position for the three curves varies considerably. A listing of the 
absorption edges determined by this technique for different deposition 
conditions is given in Table 5. From the Table, the maximxim slope 
position for films deposited under the same conditions, but with 
different thicknesses varies only slightly and as shown in Figure 6A 
the shape of the absorbance curves is similar. In contrast, the shape 
and position of the three spectrum in Figure 63 of similar thickness 
films deposited at different temperatures is altered considerably. The 
film deposited at 300° C shows very flat absorption above the band edge 
indicating a well developed band gap, but the position of the edge is 
considerably shifted to longer wavelengths which is characteristic of a 
composition change. Also, because of the inhomogeneity of these films 
there is a reduction in transmission at long wavelengths due to 



169 




Figure 63 Room temperature UV-visible absorbance spectriam of films 

deposited at LN2 temperature (LN2), room temperature (RT), 
and 300° C (300C). 



170 




gure 6A Room temperature absorbance spectra of different thickness 
films, deposited at 1 A/sec onto room temperature 
substrates . 



171 



TABLE 5 

Band Edge Position for Several Different Films 
Determined by Maximum Slope Method 



Sample 


Maximum Slope Position 
(nm) 


Energy 
(ev) 


ASDP 1 A/sec 
t = 0.5 ym 
t = 0.75 ym 
t = 1.0 ym 


507.8 
508.9 
509.8 


2.442 
2.437 
2.432 


(2.44)1 
(2.37)2 


ASDP 3 A/sec 


508.5 


2.439 




ASDP 5 A/ sec 


501.7 


2.472 




ASDP LN2 


496.5 


2.497 




ASDP 300° C 


523.1 


2.370 




FHT 500° C 
5 hr. 


505.1 


2.455 


(2.46)1 
(2.45)2 



■••Determined by maximum slope of absorption curve (Figure 79a) 
''Determined by intercept method (Figure 79b) 

Note: All films measured were approximately 1 . ym thick unless 
other wise stated. 



172 

scattering losses. For the LN2 deposited film, a large amount of band 
tailing is indicated by the higher absorbance below the band edge and 
higher absorbance above the band edge indicates the presence of 
uncompensated defects. The overall transmission of this film is 
reduced because of the presence of excess cadmium, which is the reason 
for the dark color of these films. Excess cadmium is also most likely 
responsible for the high absorbance above the band gap. A close 
relationship between the number of defects observed by TEM and the 
amount of band tailing is observed in these films. Band tailing is an 
indication of the structure of the band gap and the presence of band 
gap defects, which appear to be directly dependent on the number of 
crystallographic defects. 

The band tailing-crystal defect relationship is further indicated 
when the absorbance of films heat treated to different temperatures is 
measured. The lower heat treatment temperatures at 200° and 350° C 
shown in Figure 65 do not indicate much of a change in the amount of 
band tailing; however, the heat treatment to 500° C has reduced nearly 
all band tailing, which reveals a distinct absorption band below the 
band edge. This is an interesting result, because this is the 
temperature at which grain growth was shown to drastically increase. 
Examination of Figure 65 also shows that the higher temperature heat 
treatment results in a slight shift of the absorption edge to higher 
energy. This might be related to the slight change in composition that 
occurs at the higher heat treatment temperature. 

Because there is such a strong relationship between band tailing 
and crystal structure, examining the onset of absorption in thin films 




Figure 65 Room temperature absorbance spectra comparing as-deposited 
(ASDP) and heat treated thins. Temperature of heat 
treatment shown; all treatments for five hours. 



174 

is a useful technique for evaluating the results of different heat 
treatments. Figure 66 shows the total absorbance curve and Figure 67 
shows an expanded abscissa scale for heat treatments of 650° C 30 sec, 
650° C 4 min, and 500° C 5 hours. Figure 66 shows a structural change 
between RTA and the 500° C 5 hour heat treatment in the band tail and 
all three curves show two sub-edge features, both as shoulders above 
the sharp rise in absorption. Figure 67 indicates that furnace heat 
treating to 650° C for 4 minutes results in nearly the same low energy 
band structure as the heat treatment at 500° C for 5 hours. A rapid 
thermal anneal (RTA) film displays the same curve shape at high values 
of absorbance, but as indicated in Figure 67, there still is some band 
tailing present. 

Low temperature UV-VIS absorption 

At sufficiently low temperatures, many features of the absorption 
spectrum appear more clearly and can be related to the band structure 
in these films. Spectra were be obtained at temperatures down to 9° K 
by cooling a sample with the closed loop helium refrigerator. With a 
decrease in temperature several changes in the absorbance curve are 
observed. First the band edge shifts to higher energy, both due to a 
change in the lattice parameter and to the temperature dependence of 
the band energy. A plot of the band gap energy as a function of 
temperature is displayed in Figure 71b. 

Second, the amount of band tailing is shown to significantly 
decrease in heat treated films; however, the decrease is not as 
predominant in as-deposited thin films. The indirect transitions which 



175 



4 




4O0 420 440 4«0 480 900 920 940 960 980 800 

WAVELENGTH (nm) 



Figure 66 Absorbance spectra of different heat treatments. Furnace 
heat treatment 500° C, five hours (FHT 500); furnace heat 
treatment 650° C, four minutes (FHT 650); rapid thermal 
anneal heat treatment, 650° C, 30 seconds (RTA 650). All 
films deposited at 1 A/sec. 



176 




4«0 500 820 

WAVELENGTH (nm) 



Figure 67 Same designations as Figure 66, with expanded abscissa 
scale. 



177 

are responsible for most of the band tailing are phonon assisted 
transitions and with the lowering of the temperature, these transitions 
are no longer possible. Band tailing is not completely reduced in the 
as-deposited thin films because there are other defect states present 
in the band gap (created by the crystallographic defects) which do not 
require phonon scattering for transitions. 

The most significant change in the absorbance spectra at low 
temperature is shown to occur in films heat treated at high 
temperatures. The appearance of several sharp absorption bands which 
correspond to exciton transitions are observed. Figure 68a displays 
these bands and other changes in the absorbance spectra of a 1.0 ^m 
thick film, deposited at 1 A/sec, that was heat treated to 650° C for 4 
minutes. Shown in Figure 68b is the absorbance spectrum from a 10 \im 
thick single crystal platelet of CdS, measured under the same 
conditions as the thin film. The platelet was oriented with the c-axis 
perpendicular to the incident beam. Above the band edge, the two sharp 
peaks observed in the thin film spectrum appear similar to the platelet 
spectrum, but the features are more pronounced in the thin film 
spectrum. Also, the entire curve for the thin film is shifted to lower 
energy. The similarities of these band edge features, however, do 
indicate the presence of exciton states in the polycrystalline thin 
films . 

Further proof is afforded by plotting the absorption peaks on an 
energy scale, as shown in Figure 69a. The spectrum obtained in this 
study can be directly compared to reflection spectrum of a bulk single 
crystal CdS shown in Figure 69b. A comparison of the peak energies is 




"1 \ I I I 1 1 1 1 1 

4-iO 460 4a0 500 520 540 

WAVELENGTH (nm) 



a) 

3.8 I 




450 470 490 510 

WAVELENGTH (nm) 

b) 

Figure 68 Variation of absorbance spectra with temperature, a) 

spectrum observed at room temperature and 9° K; b) low 
temperature spectrum of thin film (TF) and single crystal 
platelet (SC). 



179 




REFLECTION SPECTRUM OF PLATELET • 4. 2 K 




(/I 

2 
UJ 
□ 



< 



a. 
o 



2. 50 2. 52 2. 54 2. 56 2. 58 2. 60 2. 62 2. 64 
PHOTON ENERGY UV) 



b) 



Figure 69 Comparison of low temperature spectra, a) absorbance 
spectrum of thin film, with energy abscissa scale; b) 
reflection spectrum of single crystal, 4.2° K.-'-^ 



180 

listed in Table 6. The absolute values of the peak energies are 
slightly different, but the energy difference between the A, B and C 
levels is nearly the same in both spectra. The difference in absolute 
values may be attributed to the same source which produces the 
difference in the absorption band edge, displayed in Figure 68b. The 
peaks are much sharper in the reflection spectra because the technique 
is very sensitive to changes in reflectivity and also because the 
reflection spectrxim was taken at 4.2° K. The broader peaks observed 
here may be due to temperature broadening, as well as orientational 
broadening in the polycrystalline films. 

The observation of exciton absorption peaks was made in all films 
heat treated to above 500° C. With the large grain growth that occurs 
with high temperature furnace heat treatments, it is not surprising 
that exciton levels should occur in a polycrystalline thin film, 
because even with a grain size of 3000 A, the band gap should be well 
established. An interesting result however, is indicated by Figure 70 
which shows exciton levels to be present in a RTA film in which the 
grain size is only 900 A. The absorption peaks are less intense and 
slightly shifted, but they are definitely present. This indicates that 
large grain growth is not required to produce polycrystalline thin 
films which display exciton levels. What is needed is only the 
annealing of certain defects. 

Another interesting observation is the variation of the exciton 
peaks with temperature. Figure 71a shows that only a slight shift and 
decrease in intensity occurs when the temperature is increased to 40° 
K, but at 100° K the A exciton peak has become a shoulder, the B and C 



181 



TABLE 6 

Comparison of Exciton Reflection and Absorption 
Peak positions 



Exciton 


Peak Position 
CdS Platelet 


Energy 
Delta 


Peak Position 
CdS Thin Film 


Energy 
Delta 


A 


2.555 




2.5A5 








0.015 




0.013 


B 


2.570 




2.558 








0.080 




0.072 


C 


2.630 




2.620 





Note: All energy values given in electron volts, eV. 



182 



FHT 




470 AlA 476 482 486 490 494 498 

WAVELENGTH (nm) 



;ure 70 Low temperature absorbance spectra of furnace heat treated 
(FHT) and rapid thermal anneal (RTA) thin film. 



183 




Figure 71 



Variation of band gap energy with temperature, a) position 
of absorbance spectrum with temperature; b) plot of band 
gap energy versus temperature. 



18A 

excitons peaks have broadened considerably. The disappearance of the A 
exciton peak at 100° K is consistent with the calculated binding 
energies of excitons, as the A exciton has a binding energy of 8 meV,^^ 
which corresponds to 92° K. Above this temperature a sharp transition 
should not be possible and we see that the peak broadens into a 
shoulder. It is interesting to note, however, that even at 300° K, as 
shown in Figure 68, an exciton-like shoulder is still present. Figure 
71a also shows the temperature dependence of the band edge, and a plot 
of this variation as a function of temperature is shown in Figure 71b. 

In the Bibliographic Review chapter (Chapter III), the description 
of the exciton states in CdS characterized the states in terms of 
crystallographic orientation and therefore their observation is 
dependent on crystal orientation with respect to the polarization of 
the probe beam. The fact that the grains in the films studied here 
have a strong preferred orientation is one reason why all three 
excitons are observed. In CdS single crystal platelets, all three 
excitons are observed when the E-field is perpendicular to the c-axis, 
but only the B and C excitons are observed with the E-field parallel to 
the c-axis. Although a circularly polarized beam is used in the Lambda 
9, the beam is correctly oriented at any one point on the film because 
the orientation of the grains can be considered to be circular about 
the c-axis pole. TEM and X-ray diffraction results indicate that many 
grains are oriented with the basal plane of the unit cell parallel to 
the film and substrate plane, so one direction vector of the unit cell 
is perpendicular to the film plane. The other two direction vectors 
which fully describe the unit cell and crystal orientation are randomly 



185 

oriented, but should be rotated about the c-axis direction vector. It 
is therefore highly probable that a grain will have the proper 
orientation to the circularly polarized beam and all three exciton 
transitions should be measured. 

Index of refraction 

Fringes of equal chromatic order. Measurement of fringes of equal 
chromatic order (FECO) was a very useful technique for determining the 
dispersion of the refractive index at wavelengths longer than the 
absorption edge. Unfortunately, equation 4.4 which was used to model 
the dispersion could not be used to calculate the refractive index at 
the absorption edge or at shorter wavelengths . This is because the 
equation over-estimates the large change in absorption coefficient that 
takes place near the band edge. As shown in Figure 9 (Chapter III) the 
refractive index for bulk CdS is found to peak at the absorption edge 
and then decrease at shorter wavelengths. The problem with equation 
4.4 is that it indicates that the refractive index converges to 
infinity at short wavelengths. This is shown graphically in Figure 72a 
and 72b, which are plots of equation 4.4, using the coefficients 
obtained from the FECO spectra of two different films. Figure 72b 
shows the calculated dispersion near the absorption edge. The Figures 
show that the curves converge at long wavelengths and give nearly the 
same value for the long wavelength limit of the refractive index. 
However, even at a wavelength 100 nm longer than the absorption edge, 
the refractive index is overestimated. Table 7 is a listing of the 
refractive index calculated at different wavelengths using equation 4.4 



186 




187 



TABLE 7 

Comparison of Refractive Index Determined by 
FECO method and Ellipsometry 



Sample 


FECO 

Refractive Index 


Ellipsometry 
Refractive Index 


ASDP 1 A/s 


3.036 


2.667 


FHT 200° C 
5 hr. 


3.120 


2.827 


FHT 350° C 
5 hr. 


3.098 


2.805 


FHT 500° C 
5 hr. 


2.874 


2.4A8 



Note: All heat treated films deposited at 1 A/sec. 



188 

and the coefficients obtained from FECO spectra of several films. The 
refractive index was calculated at a wavelength of 632 nm for 
comparison to the value of refractive index obtained by ellipsometry, 
listed in the last column of the Table. Table 7 shows that the FECO 
method for determining refractive is only valid at long wavelengths. 

Ellipsometry measurements. By using the graphical method 
described in Appendix B to determine the refractive index from the 
polarization angles measured with the ellipsometer , very accurate 
results could be obtained. These results are useful for quantitative 
comparison of different films, unfortunately this comparison could only 
be made at one wavelength; i.e. 632 nm, the wavelength of the HeNe 
laser used with the ellipsometer. Table 7 shows there is a significant 
change in refractive index when a thin film is heat treated. This 
should be expected because the density of the film changes 
considerably, particulary from high temperature heat treatments. The 
results show that ellipsometry is sensitive to the slight differences 
in microstructure of thin films deposited at different rates. In 
addition to these advantages, the technique is also very useful for 
refractive index and thickness determinations on very thin films, which 
cannot be measured with the FECO technique. 

Absorption coefficient. A very close approximation to the 
wavelength dependence of the absorption coefficient can be made by 
taking the absorbance spectra and applying a few simple relationships. 
First, the extended equation for the transmission of light through a 
thin film supported on a substrate (equation k.l. Chapter III) which 



189 

takes into account the reflections that take place within the film and 
the resulting interference can be written as 



T = __^__l__ (5.1) 
1 - B e"2 ° ^ 



where 

A = ( 1 -R^) ( 1 - R^) ( 1 - R^) ( 1 - R2R3)' ^ 
and 



B = ( R^R^ + R^R^ ) ( 1 - R^ 



Equation 5.1 can be simplified and written as 



T = ^ (5.2) 

at „ -a t 
e -Be 



and the inverse is given by 



1 lat B-at /roN 
= e - e . (5.3) 

T A A 



If the absorption coefficient a is assumed to be very large (a ~ 10^) 
then the second term in equation 5.3 is essentially zero. Since the 
absorbance is defined as the natural logarithm of inverse transmission, 
then the absorption coefficient can be calculated from the absorbance 
spectra by applying 



190 



ABSORBANCE + In A . ^ , x 

a = . (.5.4; 

t 

The term In A represents the losses due to reflection interference and 
the value can be determined from the absorbance spectra of different 
thickness films, such as Figure 64, or a close approximation can be 
made by using the value of absorbance below the band edge, such as the 
value at 580 nm in Figure 6A. The absorption coefficient spectra as a 
function of energy obtained by this calculation is shown in Figure 73a 
for both an as-deposited film and a heat treated film. The value of 
the optical band gap is given by either the maximum in slope of this 
curve, or by plotting the square of a and extrapolating the linear 
portion of the curve to the intersection of the x-axis. The value of 
the X-axis intercept is the band gap energy. This plot is shown in 
Figure 73b for the same two films and a listing of other films and a 
comparison of the different techniques for calculating the band gap is 
presented in Table 5. 

The above Figures and Table when compared to the data presented in 
the Bibliographical Review chapter show that the thin films produced in 
this study exhibit similar results to single crystal properties. This 
is another indication of the high quality of films which can be 
produced by the deposition process and subsequent heat treatments that 
were used in this study. 
Photoluminescence and Raman Spectroscopy 

Photoluminescence offers a complementary analysis to absorption 
measurements. While the optical transitions involve similar states. 



191 

5.5 -1 




0-5 -\ 1 1 1 1 r~ — -1 — r 1 1 1 — ~i 1 r- 

1 1.4 1.8 2.2 2.6 3 3A 3.B 

ENERCr (•V) 



a) 





15 -1 








14 - 








13 - 






1 


12 - 






E 
u 

^> 


11 - 






N 
< 

9 ' 

ti 

S3 


10 - 
9 - 

a - 

7 - 


ASPD /j 


FHT 


ABSORPTION 


6 - 
5 - 
4 - 

3 - 


/ 1 
// / 

/ / / 






2 - 






1 - 








- 


r 1 1 ■ I ' r- 


1 1 1 1 1 



2 2.2 2.4 2.6 2.B 3 

ENERGY (eV) 

b) 

Figure 73 Alternate methods for determining the value of the optical 
band edge, a) plot of absorption coefficient calculated 
from Equation 5.4 versus photon energy, point of maximum 
slope gives the band energy; b) plot of versus photon 
energy, extrapolation to = gives band gap energy. 



192 

they are not subject to the same selection rules. For direct band gap 
semiconductors, radiative recombinations generally occur near k=0. 
Just as all the different defect states in the band gap lead to 
discrete lines, or bands, or tailing in absorption spectra, they can 
also lead to photoluminescence spectra which are indicative of these 
defects. As described earlier, the defect states that are of the 
greatest interest to this study are the exciton states and 
photoluminescence is an excellent technique for studying their 
occurrence and behavior. 

Use of the optical multichannel analyzer (OMA) and the Raman 
spectrometers permitted a wide variation in the resolution and spectral 
window over which the photoluminescence and Raman scattering of thin 
films could be measured. For analysis of certain thin films which 
displayed very low intensity, wide band photoluminescence, the OMA 
could be used with a very low dispersion grating (152 lines/mm). While 
giving very poor spectral resolution this arrangement still permitted 
detection of very low light levels. To measure high intensity, narrow 
bandwidth peaks, the low dispersion grating was replaced with a very 
high dispersion 2400 lines/mm grating. As explained in the 
Experimental Method chapter (Chapter IV) this grating permitted a 
bandpass resolution of 0.9 nm, but a spectral window of only 20 nm. To 
assure calibration and permit an even higher bandpass resolution, the 
Raman spectrometer was used. By controlling the slit widths of the 
monochromater, resolutions down to 0.05 nm could be obtained. 



193 

Photoluminescence spectra 

As-deposited thin films. Thin films which were deposited at room 
temperature were found to only give very low intensity, broad band 
photoluminescence. The defect structure of the band gap that led to 
the band tailing observed in the absorbance spectra of these films is 
also responsible for the low intensity observed in these measurements. 
These defects states provide non-radiative processes for the relaxation 
of excited states, so that the photoluminescence spectra have very low 
intensities. Because there are many different states with slightly 
different positions in the band gap, a broadening of the luminescence 
also occurs. A typical spectrum of films deposited at low rates, 
obtained at room temperature with the OMA and the low resolution 
grating is shown in Figure 74. This is an expanded intensity scale so 
the signal to noise ratio is low. The sharp peak at A57 nm is the 
laser line used to excite the sample. Usually, the three different 
broad bands observed in this spectr\im would occur in all films 
deposited at room temperature, but the intensity of the different bands 
would vary. For example, the higher intensity band centered at 535 nm 
was found to be of much lower intensity in films deposited at higher 
rates and it was totally absent in films deposited at 5 A/sec. The 
energy of the 535 nm band corresponds to direct transitions in the band 
gap and is the "green-edge" emission that was discussed in the 
Bibliographical Review chapter (Chapter III). The radiative 
transitions from conduction to valance band or from shallow donor to 
shallow acceptor impurity levels are characterized by this emission. 
The two low intensity broad bands centered at 670 nm and 750 nm are 



194 




Figure 74 Low resolution photo luminescence spectrum acquired with 
optical multichannel analyzer (OMA). 



195 

associated with deep impurity level luminescence. The relative 
intensities of these two bands could be altered with slight changes in 
the geometry of the detection system. The width of these bands made it 
difficult to center the grating so that the diode array was evenly 
illuminated. This is one example of problems associated with a linear 
diode array detection system. If the array is not evenly illuminated 
the relative intensities displayed in the emission spectra will not be 
proportional. 

Films deposited at LN2 temperatures only displayed the longer 
wavelength bands and at an even lower intensity level. The green edge 
emission is not observed in these films because the band structure is 
not well defined, due to the large number of crystallographic defects. 
The long wavelength bands are observed because deep level traps do not 
need a well established band gap to cause radiative transitions. They 
can be likened to impurity color centers, which are associated with a 
single impurity atom, or in this case with a vacancy or interstitial 
atom associated with an impurity. The identity of this impurity is 
unknown, but some authors have reasoned that it could be a halogen atom 
such as chlorine. ■^■^»^^ 

Heat treated thin films. A very significant change in the 
photoluminescence of thin films occurs in films which are heat treated 
at high temperatures, even for relatively short times. The intensity 
of the green edge emission increases over 5x for the same excitation 
intensity and the peak position shifts to a higher energy. The 
relative intensity of an as-deposited thin film and a film heat treated 
to 500° C for 30 minutes are shown in Figure 75 and the relative peak 



196 




Figure 75 Low resolution photoluminescence spectra of heat treated 

and as-deposited thin films, showing relative intensities. 



197 




Figure 76 Same spectra as Figure 75, relative peak positions. 



198 

positions are shown in Figure 76. Both of these changes occur because 
the band gap is better defined after the annealing heat treatment. 
With this change in band structure, the band edges are sharper, which 
shifts the energy and increases the probability of a radiative 
transition. Figure 75 also shows that the intensity of the laser line 
has increased significantly, which is most likely due to an increase in 
scattering of the laser line. 

All films which were heat treated to high temperatures displayed a 
high intensity edge emission peak and the peak position was found to be 
nearly the same. One difficulty with determining peak position, 
however, was that absorption of the laser line would cause enough 
heating of the film to shift the position of the peak to lower energy. 
A graph of the shift in peak position with incident laser power is 
displayed in Figure 77. At very low powers only a small shift occurs, 
but at higher incident powers the shift is significant. At even higher 
incident powers the focussed laser beam would cause vaporization of the 
thin film. Very low incident laser powers were therefore used to study 
the room temperature photoluminescence of thin films, although low 
power excitation reduced the intensity of the emission. 

The problem with power dependent peak shift was reduced 
considerably by cooling the thin film with the cold finger and 
acquiring photoluminescence spectra at low temperature. As with UV- 
visible spectroscopy, several significant changes in thin film 
photoluminescence are observed when the film is cooled to 9° K. First, 
the green edge emission band increases in intensity and shifts to a 
higher energy corresponding to the increase in band gap energy with 



199 




i h 1 h 1 1 1 1 1 1 1 1 

510 512 514 S16 518 520 522 

PEAK POSmON (nm) 



Figure 77 Graph of band edge luminescence peak position versus 
incident power. 



200 

decrease in temperature. Second, the peak position becomes less 
sensitive to incident power levels, i.e. no shift occurs unless very 
high powers are used. Finally, the most significant change which 
corresponds to the exciton absorption peaks seen in the UV-visible 
spectra, is the appearance of the "blue-edge" emission peak. Because 
of the narrow bandwidth of this peak the high resolution grating had to 
be used in the OMA monochromater . A typical photoliominescence spectrum 
of the blue edge emission exhibited by a thin film which also displayed 
the measurable absorption peaks is shown in Figure 78 and a composite 
spectra showing the relative intensities of the two emission bands is 
shown in Figure 79. The sharp peak at A92.3 nm in Figure 78 
corresponds to one of the exciton transitions, although it is not clear 
which one. The first two absorption peaks in the absorbance spectrum 
separated = 3 nm should exhibit two photoluminescence peaks, but only 
one can be seen here. The photoluminescence spectrum obtained from the 
same single crystal platelet used in the absorption measurements 
(Figure 68b) is shown in Figure 80, compared with the blue edge 
emission measured in the film described above. The platelet displays 
both the 1^ and I2 bound exciton peaks and as can be seen, the 
transitions are much sharper and shifted to energies higher than the 
photoluminescence exhibited by the thin film. The shift in position is 
identical to that seen in the absorption edge displayed in Figure 67b. 
The shape of the thin film photolximinescence is similar to the 
platelet; the high energy side is steeper. The peak width of the thin 
film emission is nearly five times wider. In other work on both on 
bulk single crystal CdS and CdS epitaxial thin films, exciton 



201 




466 488 480 402 494 48fi 486 500 502 

Vavalangth (n«) 



Figure 78 High resolution OMA spectrum of heat treated thin film 
9°K, showing exciton photoluminescence. 



202 




400 485 SOO SOS 310 515 520 

Wavalangth irm) 



Figure 79 Composite low temperature OMA spectrum showing relative 
intensities of band edge and exciton emissions. 



203 



3000 



2500 



2000 



ISOO 



IQOO 



500 




478 



482 



Wavalangth (nn) 



Figure 80 Low temperature OMA spectra of thin film and single cryst 
platelet, showing relative peak positions. The bound 
and I2 excitons are labeled on the platelet 
photo luminescence . 



20A 

transitions were exhibited by very sharp emission bands, although these 
measurements were made at lower temperatures. The measurements of 
single crystal platelets made here show that the exciton emissions are 
just as sharp at 9° K. This rules out the possibility that the 
emissions measured from thin films are temperature broadened. One 
possible reason for the observed broadening may be orientational 
broadening or the presence of grain boundaries. All materials which 
have displayed sharp exciton photoluminescence have been essentially 
single crystals. The impurity centers near grain boundaries which 
excitons are bound to may vary in energy slightly from those impurity 
centers within the interior of grains. This might produce the 
broadening of the exciton photoluminescence observed in polycrystalline 
thin films. 

The photoluminescence spectra displayed here exhibits another 
difference from the absorbance spectra. The three absorbance bands 
were characterized as free exciton transitions, based on their position 
and shape. The blue edge emission exhibited by thin films, however, is 
more characteristic of bound exciton transitions. As previously 
explained, these are excitons which are bound to impurity sites. 
Usually the impurity is a neutral acceptor or neutral donor, which give 
rise to an I]^ or an I2 bound exciton, respectively. It is certainly 
reasonable to expect that these types of impurity centers exist in the 
thin films studied here. Sulfur interstitial atoms or cadmium vacancy 
atoms could act as donor impurity sites, which correlates well with the 
single peak exhibited in the blue-edge emission of thin films. 
However, compared to the single crystal platelet photoluminescence, the 



205 

blue-edge emission is certainly broad enough for both bound excitons to 
be present. The bound exciton is a much stronger transition, so it 
should be easier to detect in photoluminescence measurements. The free 
exciton emission may be too low in intensity to detect. For absorption 
measurements the cross section of the free exciton is much larger than 
the bound exciton, because it does not have the same momentxim 
constraints as the bound exciton. Therefore, the absorption spectra of 
free excitons should be broad and the bound exciton should exhibit a 
sharp absorption band. Estimations show that the UV-VIS 
spectrophotometer may not have a high enough resolution to observe the 
bound exciton absorption peaks. Photoluminescence on the other hand 
does not have the same momentum constraints, therefore the emissions 
will have different characteristics. Since the bound exciton has both 
a higher oscillator strength and more efficient radiative conversion, 
bound excitons should be more predominant in photoluminescence spectra. 

The blue edge emission displays a temperature dependence similar 
to optical absorption. This is shown in Figure 81. As the temperature 
is increased, the photoluminescence peak shifts to longer wavelengths 
(lower energy) and decreases in intensity. As with the UV-visible 
spectra, the peak is still present at A0° K, but at 100° K it becomes 
indistinct. The Figure also shows that with an increase in temperature 
the intensity of the green edge emission peak decreases. The decrease 
occurs because as the temperature is increased, more phonons become 
available to cause non-radiative transitions. 



206 




Figure 81 Temperature dependence of blue edge photoluminescence. 



207 

Raman spectroscopy 

The photoluminescence measurements described above were conducted 
by exciting the films with a laser beam of energy much greater than the 
band gap. Excited states which are generated must first drop to a 
lower energy corresponding to either an impurity level, or an exciton 
level by way of a non-radiative transition before a radiative 
transition can occur. When a excitation energy is used that is very 
close in energy to the excited state, the energy can be absorbed and 
then scattered by a phonon state. The scattering by the phonon results 
in the re-emission of the light at a different frequency. Because the 
excitation energy is very close to the excited state level, a resonance 
effect takes place and an enhancement of the scattering results . This 
process is called resonant Raman scattering and in thin films it was 
found to be a very pronounced effect when a laser energy near the 
energy of the exciton level was used to excite the film. Resonant 
Raman scattering by the longitudinal optical (LO) phonon of the 488 nm 
laser line would result in a series of equally spaced peaks that were 
shifted ~ 300 cm"-'- from the laser line. These peaks were very intense 
and could be measured with either the OMA or Raman spectrometers. A 
typical spectriim of the first two LO phonon peaks which was acquired 
with the OMA and plotted in terms of Raman shift from the laser line is 
shown in Figure 82. This spectra was acquired with similar scan 
parameters and laser intensity as photoluminescence spectra, but the 
intensity of the ILO phonon is much greater than any photoluminescence 
peak. The temperature dependence of this emission is shown in Figure 
83. A similar low temperature spectrum obtained with the Raman 



208 




igure 82 Longitudinal optical (LO) resonant Raman peaks measured 
with OMA. 



209 




Figure 83 



Temperature dependence of resonant Raman peaks 



210 




Figure 84 Same LO peaks displayed in Figure 82, shown here measured 
with the Raman spectrometer . 



211 

spectrometer and plotted in waveniambers is displayed in Figure 84. The 
same laser power was used to excite the sample, but as can be seen a 
much lower intensity is measured. This indicates how much lower the 
light gathering ability of the Raman is compared to the OMA. The 
spectra however, also show that the resonant Raman effect is strong. 
Laser powers for normal Raman spectroscopy typical are in the range of 
a few watts. Even with this high power the Raman scattered peaks are 
very low in intensity. The laser power used to excite the samples 
measured in this study was only one milliwatt! The high intensity of 
the ILO peak is due to a strong interaction with the exciton state. 
This is further proof of the existence of exciton levels in the 
polycrystalline thin films. 

COSAD Optical Properties 

The microstructure analysis described in the physical properties 
section has indicated that it is possible to produce structures with 
finely dispersed CdS crystallites in a glass matrix thin film. The 
important question about these structures is whether quantum 
confinement occurs, which would be indicated by a shift in the optical 
absorption edge to higher energies. Unfortunately, a shift could also 
be produced by a change in composition. Since COSAD 1 films were made 
with a high CdS content, which once heat treated, leads to structures 
too large for quantum confinement, it would be expected that any shift 
in the band edge would be due to compositional effects. On the other 
hand C0SAD2 films should exhibit a shift due to quantum confinement, 
because of the small particle size displayed in these films. 



212 




WAVELENGTH (nm) 



Figure 85 Room temperature absorbance spectrum of COSALl thin film. 

As-deposited (ASDP) and after heat treatment to 650° C, 60 
minutes (FHT). 



213 

A typical absorbance spectrum of an as-deposited COSADl film is 
shown in Figure 85. The high absorption tail at short wavelengths is 
due to the glass matrix. This type of absorption is an indication of a 
nonstoichiometric oxygen content and has been shown to occur in other 
glasses which were sputter deposited with an argon plasma. The 
spectrum of the as -deposited COSAD film shown in the Figure exhibits no 
appearance of a band edge, which is inconsistent with the 
raicrostructural analysis that indicated a semi-crystalline structure. 
Also shown in Figure 85 is the absorbance spectrum of a COSADl type 
film heat treated to 650° C in an air atmosphere. This spectrum, at 
first, certainly appears to be the result of quantum confinement; the 
band edge has shifted to A60 nm (2.70 eV) and the exciton shoulder is 
more pronounced. Microstructure analysis on this type of film however, 
indicated a crystallite size much too large for quantum confinement, 
which means that this shift in band edge energy is most likely a 
crystal composition effect. Further proof that this is a chemical 
shift is shown in Figure 86 which displays the spectrum of a COSADl 
type film heat treated in an argon atmosphere. The band edge shift is 
less and the exciton shoulder is less pronounced. 

It is probably not correct to call the feature on the absorption 
edge an exciton shoulder, as the shape of this feature does not change 
considerably as the temperature is decreased. In pure films of CdS the 
room temperature shoulder resolved into two sharp absorption bands at 
low temperatures. As shown in Figure 87 the shoulder does not 
appreciably change shape at 9° K and there is only a small shift in the 
band edge. 



214 




Figure 86 



Room temperature absorbance spectrum of COSADl thin film 
heat treated in air (AR) and argon (AG), at 600" C, for 30 
minutes. 



215 



0.7 




420 460 500 

WAVELENGTH (nm) 



Figure 87 Low temperature absorbance spectrum of COSADl thin film. 



216 

A much different absorbance spectrum is displayed by the C0SAD2 
type films. Shown in Figure 88 are the spectra of both an as-deposited 
and a heat treated thin film. Unlike COSADl films this type of film 
shows the appearance of a band edge which is shifted to a very high 
energy, even in the non-heat treated state. This is an interesting 
result, because although the microstructure of this type of film is 
very small, it appeared to be amorphous. Heat treatment to 600° C in 
an argon atmosphere produced only a small change in the edge position, 
but it was still shifted to a higher energy than COSADl films. The 
large shift in band energy must be related to a confinement process. 
Another interesting result is indicated by the absorbance scale, which 
is an order of magnitude less than the scale used to measure COSADl 
films. This is an indication that there is considerably less CdS in 
the structure of C0SAD2 films. 

Unfortunately no photoluminescence spectra could be obtained from 
either type of COSAD film. This may be due to a dilution effect, due 
to both a low CdS concentration in these films and a small thickness. 
Also, because the band edge was shifted to such a high energy in the 
C0SAD2 films it was not possible to excite the sample, even when using 
the highest energy argon laser line. 



217 



UJ 

o 
z 



IT 

o 

m 
< 




JOO 340 380 420 460 

WAVELENGTH (nm) 



500 



S40 



S80 



Figure 88 Room temperature absorbance spectrum of C0SAD2 thin film. 

As-deposited (ASDP) and after furnace heat treatment at 
650° C for ten minutes (FHT). 



CHAPTER VI 
CONCLUSIONS 



1. RF magnetron sputtering of cadmium sulfide has proven to be a 
valuable deposition technique for producing high optical quality thin 
films. The as-deposited thin film structure can be controlled by a 
variation of the deposition rate and substrate temperature. Unique 
structures can be produced by deposition onto substrates held at LN2 
temperatures. A low density columnar structure is produced in which 
small microcrystallites are embedded in a semi-amorphous matrix. 
Electron diffraction experiments indicate that the microcrystallites 
are highly faulted, which leads to a degradation of the optical 
properties. The crystallographic defects provide defect states in the 
band gap which are indicated by the large amount of band tailing 
observed in optical absorption spectra. Optical properties are further 
degraded by the presence of excess cadmium. The large number defects 
responsible for band tailing provide non-radiative transitions so that 
band edge photoluminescence is not observed in these films. 

2. High density fine grained polycrystalline films result when 
sputtered material is deposited onto substrates held at room 
temperatures. Many of the grains appear to be faulted, with the 
presence of stacking faults and microtwins. The number of faults 
increases with the deposition rate, therefore the faults are related to 
the growth rate of the thin film. Electron and X-ray diffraction 

218 



219 

experiments indicate the film has a very strong preferred orientation, 
with the basal plane parallel to the substrate plane. It is not clear 
if this orientation is nucleation or growth controlled. The close- 
packed plane is the slow growth direction, which would indicate the 
preferred orientation is controlled by nucleation. This same 
orientation, however, was observed over a substrate temperature range 
of nearly 500° K and with various substrate materials. It is unusual 
that a nucleation controlled process would extend over such a large 
temperature range, although with this material the nucleation range may 
be very wide. A combined effect may actually control the process if 
the crystallites can be modeled as thin disks. Once the initial 
orientation is nucleated, only a small step in the z-direction could 
cause a large increase in growth in the lateral direction. X-ray line 
shifts indicate films deposited at high rates contain large residual 
tensile stresses, which leads to their exfoliation when exposed to the 
atmosphere. Optical absorption spectra indicate some band tailing to 
be present in room temperature deposited films, which is related to the 
crystallographic defects that are present. The band structure in these 
films is defined well enough so that band edge photoluminescence is 
exhibited; however, the large number of defects present in the band gap 
leads to a decrease in the photoluminescence yield. 

3. When CdS is deposited onto substrates held at high temperatures a 
reduction in the nvunber of crystallographic defects is observed. With 
higher substrate temperatures, adsorbed atoms have more energy to move 
around and find favorable positions with which to bond. The preferred 
orientation is still maintained, however, other hexagonal orientations 



220 

can be observed because other growth planes may become active at 
higher temperatures. With the decrease in the number of 
crystallographic defects, band tailing is reduced indicating a better 
defined band gap; however, a shift in the absorption edge is observed. 
The shift is due to a change in composition, as these films are found 
to be nonstoichiometric. A higher sulfur content in these films occurs 
because the sticking coefficient of cadmium is reduced at higher 
substrate temperatures. Due to large nonuniformities in the 
composition across the film area a degradation in the optical 
properties at wavelengths longer than the band edge occurs. 

4. To produce the highest quality thin films for optical applications 
a post deposition heat treatment must be instituted to remove defects 
created by the deposition and growth processes. Heat treatments of 
less than 500° C cause a small change in grain size which appears to be 
linear with the increase in heat treatment temperature. Low 
temperature treatments also lead to a small change in the optical 
properties. Band tailing is still present and photoluminescence yield 
is still low. With heat treatments to 500° C and above, a dramatic 
increase in grain size occurs. Higher temperatures may provide the 
activation energy required for impurity assited diffusion at the grain 
boundaries. Associated with this grain growth is a reduction in the 
amount of band tailing and a large increase in the photoluminescence 
yield. These results are due to a removal of crystallographic defects 
with grain growth, which are related to defects in the band gap. 

5. For the first time absorption bands and photoluminescence 
associated with exciton states have been observed in polycrystalline 



221 

thin films. Associated with the changes in microstructure that occur 
upon heat treatment to high temperatures is the appearance of exciton 
levels. These levels appear as sharp absorption bands or as low 
intensity blue edge photoluminescence. There is, however, considerable 
broadening of the blue edge photoluminescence associated with the 
exciton levels. The shape of the emission peak is similar to that 
observed in single crystal platelets; however, it is = 5 times wider. 
This broadening may be due to orientation effects or the presence of 
grain boundaries which provide different environments for the 
annihilation of excitons to produce the photoluminescence. 

6. Heat treating films by rapid thermal anneal (RTA) produces films 
which also exhibit exciton levels. The grain size of these films is 
three times smaller than furnace annealed thin films, which indicates 
that for excitons to be present in sputter deposited thin films, only 
the annealing of certain defects is required, not large grain growth. 
The actual defects which must be removed are not known, but they must 
be those associated with band tailing, as the appearance of exciton 
levels is coincident with the removal of band tailing. From TEM 
micrographs of unheat treated films, these defects most likely are the 
stacking faults associated with microtwins. 

7. Thin films were found to be nonstoichiometric, particularly after 
high temperature heat treatments where the sulfur content was found to 
increase to approximately 4%. The increase in sulfur is obviously due 
to a loss of cadmium, which has a much higher vapor pressure than 
sulfur at the heat treatment temperature. Only a small portion of the 
excess sulfur may exist interstitially, as the indication from other 



222 

chalcogenide compounds is that this type of defect would be limited to 
less than 1%. Calculations show that if all the excess sulfur were 
present at the grain boundaries as a thin layer, then this amount of 
sulfur would be six to seven monolayers, which corresponds to a grain 
boundary layer = lA A thick. For an average grain size of 3000 A, this 
is not an unreasonable thickness for grain boundaries. The uneven 
contrast observed in X-ray maps of sulfur in heat treated films 
indicates that there is an uneven distribution of sulfur and therefore 
it may exist as small pockets, instead of a thin evenly distributed 
second phase. The presents of this second phase does not effect the 
optical properties significantly, but it may be responsible for the 
large resistivity observed in high temperature heat treated thin films. 
8. The limited study on COSAD films indicates that it is possible to 
produce a structure where very fine particles of a semiconductor are 
embedded in a glass matrix. By variation of the amount of CdS that is 
co-deposited a wide variation in the structure of the film can be 
accomplished. High amounts of CdS produce a film which readily phase 
separates during examination in the TEM. When this type of film is 
heat treated to high temperatures a very distinct two phase structure 
results where the CdS is dispersed as a bimodal distribution of 
spherical particles. The larger particles have an average size of 500 
A, and the smaller particles are sized at 200 A. When very low 
concentrations of CdS are co-deposited, then a structure which does not 
phase separate results. After heat treatment, very small particles of 
less than 70 A result. 



223 

9. Shift of the absorption edge in COSAD films appears to be mostly a 
chemical shift and not due to quantiim confinement. Heat treatments to 
the same temperatures and times, but in different atmospheres result in 
different shifts of the band edge. Heat treatments in an oxidizing 
atmosphere produced a larger shift in the band edge than heat 
treatments in an inert atmosphere. This could be due to the formation 
of cadmium oxide, which has a larger band gap energy than cadmium 
sulfide. The large shift of the band edge, however, cannot be entirely 
due to the formation of an oxide, as the edge observed in films treated 
in inert atmospheres is still considerably shifted from the energy of 
pure CdS. There may be other unknown chemical effects or some 
confinement process occurring. Another possible reason for the shift 
could be due to surface states, which would be enhanced by the large 
difference in dielectric constant at the interface and by the large 
stress created by the high curvature of the spherical particles. 

10. The broad shoulder on the top of the absorption edge, which 
appears similar to the room temperature exciton shoulder observed in 
pure films of CdS is of unknown origin in COSAD films. The shoulder 
does not change shape with decreases in temperature, although there 
still could be considerable orientational broadening even at low 
temperatures. Unfortunately no photoluminescence was observed in COSAD 
films primarily due to a dilution effect, arising from the thickness of 
the films and the relatively small amount to CdS. Photoluminescence 
would have indicated the presence of excitons. 



CHAPTER VI 
FUTURE WORK 



There are several aspects of cadmium sulfide and COSAD thin films 
which require further study and investigation. 

Obviously, the first study is to make a nonlinear optical 
measurement to determine if the exciton states in the pure films of CdS 
can be saturated to produce an effect that is comparable to the large 
nonlinear refractive index observed in single crystal platelets. Since 
the films are of high enough optical quality to act as thin film Fabry- 
Perot interferometers, a very simple intensity saturation experiment 
could be conducted. ^'^ The only difficulty in this experiment is the 
need for a high resolution tunable dye laser which can be operated in 
the wavelength region of 488 nm, the wavelength required for exciton 
saturation.^ A shorter wavelength, high speed pulsed laser could also 
be used to measure the nonlinear absorption, (due to a free carrier 
plasma) by measuring the temporal waveform of a transmitted pulse. 
If this pulsed laser could be used with the dye laser, the change in 
the waveform as the laser is tuned near the exciton level would provide 
valuable information. Finally, if a nonlinear intensity saturation was 
observed, then a pump-probe experiment could be carried out to 
determine the switching speed of the thin film Fabry-Perot. 

Further work on the stoichiometry as a function of heat treatment 
atmosphere should be carried out to determine the influence of sulfur 

22A 



225 

concentration on the band structure. Heat treatments in equilibrium 
sulfur or cadmium atmospheres may produce stoichiometric thin films. 
It would be interesting to know if the electrical conductivity and 
photoluminescence change when the excess sulfur is removed. Other post 
deposition treatments which should be further investigated are heat 
treatments by RTA, to determine if very small, defect free crystals 
could be produced in a thin film. 

A large research effort should be directed towards further 
investigations of COSAD thin films, since these films appear quite 
promising for quantum confinement devices. Studies of the deposition 
conditions and heat treatment schedules on the microstructure 
development in these films could provide both a parallel confirmation 
of the effects observed in bulk filter glasses and permit a further 
understanding of the confinement processes that take place in these 
materials. Photoluminescence studies of thicker films would positively 
identify a confinement process, by correlation of the blue shift with 
particle size. If well characterize COSAD thin films could be 
produced, nonlinear optical measurements are easily made because these 
films on the proper substrate should act as low loss planar waveguides. 
When prisms are used to couple light into and out of thin film 
waveguide, the angle at which the light couples is dependent upon the 
refractive index of the film. The nonlinearity of these films could be 
determined by measuring a power dependent coupling angle. If these 
experiments proved successful, then the next step would be either to 
attempt logic gate experiments by utilizing small scale integration, or 



226 

examine a different semiconductor to determine if a confinement process 
could be induced. 



APPENDIX A 

BASIC PROGRAM USED FOR EXTERNAL CONTROL OF LAMBDA 9 



1000 REM LAMBDA 9 COMMUNICATIONS PROGRAM 
1010 REM VER. UWIS6.BAS 

1015 REM WRITE COMMAND USED TO OUTPUT DATA TO FILE NAME *.PRN 
1020 REM FOR ASYST PROGRAM INPUT AND LOTUS WORKSHEET 
1030 CLEAR: CLOSE: KEY OFF:CLS 
lOAO DIM X(3000) 

10A5 REM ARRAY MUST BE LARGE ENOUGH TO ACCOMMODATE ENTIRE SCAN 

1050 INPUT "TRANSMISSION OR ABSORBANCE SCAN T/A ";ANS$ 

1060 IF ANS$="T" THEN 1070 ELSE 1080 

1070 SB=0: PRINT "TRANSMISSION SCAN":GOTO 1090 

1080 SB=5: PRINT "ABSORBANCE SCAN" 

1090 INPUT "SELECT ABSCISSA MAX ";AH 

1100 INPUT "SELECT ABSCISSA MIN ";AL 

1110 INPUT "SELECT SCAN SPEED ";SS 

1120 INPUT "SELECT DATA INTERVAL ";DI 

1130 INPUT "SELECT RESPONSE ";FR 

11 AO INPUT "SELECT SLIT WIDTH ";SL 

1150 SL=SL*100:REM CONVERT SLIT SETTING 

1160 REM CALCULATE NUMBER OF DATA POINTS 

1170 XPTS=(AH-AL)/DI 

1180 INPUT "SELECT NORMAL (1) OR EXTENDED RANGE (2) ";RA 

1190 INPUT "SELECT CHART REGISTRATION: OFF(O) .SERIAL DASH (1), 

OVERLAY DASH (2)";CR 
1200 IF CR=0 THEN 1260 

1210 INPUT "SELECT PRINTER FUNCTION: OFF (0),GRID + SCALE (1) " 

1220 INPUT "SELECT PEN LINE MODE (1-4) ";PE 

1230 INPUT "SELECT RECORDER FORMAT (nm/cm) ";RS 

12A0 INPUT "SELECT ORDIANTE MAXIMUM ";MX 

1250 INPUT "SELECT ORDINATE MINIMUM ";MI 

1260 IF CHG$="Y" THEN 1350 

1270 REM OPEN COMMUNICATIONS PORT 

1280 OPEN "COM1:A800,E,7,1" AS//1 

1290 PRINT //1,"$RE 0" 

1300 PRINT //1,CHR$(17) 

1310 INPUT //1,D$ 

1320 PRINT //1,"$PA 2" 

1330 INPUT //1,D$ 

13A0 PRINT D$ 

1350 REM SET UP INSTRUMENT 

1360 PRINT //1,"$SB" + STR$(SB) 

1370 INPUT //1,D$ 

1380 PRINT //l."$AH" + STR$(AH) 

1390 INPUT //1,D$ 

lAOO PRINT //1,"$AL" + STR$(AL) 



227 



228 



lAlO INPUT //1,D$ 

1420 PRINT #1,"$SS" + STR$(SS) 

1430 INPUT //1,D$ 

1A40 PRINT //1,"$DI" + STR$(DI) 

1A50 INPUT //1,D$ 

1460 PRINT //1,"$FR" + STR$(FR) 

1470 INPUT //1,D$ 

1480 PRINT //1,"$SL" + STR$(SL) 

1490 INPUT //1,D$ 

1500 IF CR=0 THEN 1590 

1510 PRINT //1,"$CR" + STR$(CR) 

1520 INPUT //1,D$ 

1530 PRINT //1,"$RS" + STR$(RS) 

1540 INPUT #1,D$ 

1550 PRINT //1,"$PE" + STR$(PE): INPUT //1,D$ 

1560 PRINT //1,"$PR" + STR$(PR): INPUT //1,D$ 

1570 PRINT //1,"$MX" + STR$ ( MX ): INPUT //1,D$ 

1580 PRINT //1,"$MI" + STR$(MI): INPUT //1,D$:GOTO 1600 

1590 PRINT //1,"$CR 0": INPUT //1,D$ 

1600 PRINT #1,"$RA" + STR$(RA) : INPUT //1,D$ 

1610 PRINT CHR$( 10); "CONFIRM SELECTED VALUES ON LAMBDA 9 SCREEN" 

1620 INPUT "CHANGE ANY VALUES Y/N ";CHG$ 

1630 IF CHG$="Y" THEN 1640 ELSE 1650 

1640 CLS:GOTO 1050 

1650 PRINT #1,"$SC" 

1660 INPUT //1,D$ 

1670 PRINT D$ 

1680 REM PRINT DATA 

1690 FOR 1=1 TO XPTS+12 

1700 INPUT //1,X$ 

1710 S$=LEFT$(X$,1) 

1720 S=VAL(S$) 

1730 IF S=0 THEN S=LEN(X$) ELSE 1750 
1740 X1$=RIGHT$(X$,S-1) :X$=X1$ 
1750 X(I)=VAL(X$) 
1760 NEXT I 

1770 INPUT "STORE DATA ON DISK Y/N ";ANS$ 
1775 IF ANS$="Y" THEN 1780 ELSE 1910 

1780 INPUT "INSERT DATA DISK IN DRIVE A AND TYPE R ";R$ 
1790 IF R$="R" THEN 1800 ELSE 1780 

1800 INPUT "NAME OF FILE (MAXIMUN OF 8 CHARACTERS)" ;FILN$ 
1810 IF LEN(FILN$)>8 THEN 1800 
1820 IF LEN(FILN$)=0 THEN 1800 
1830 OPEN "0",//2,"A:"+FILN$+".PRN" 
1840 N=XPTS+11 

1850 PRINT #2,AH:PRINT //2,AL:PRINT //2,DI 
1860 FOR 1=1 TO 11:WRITE //2,X(I) :NEXT I 
1870 IF SB=0 THEN SCALE=200 ELSE 1890 

1880 FOR J=ll TO N:WRITE #2,X( J)/SCALE:NEXT J:GOT0 1910 
1890 SCALE=10000 



229 



1900 FOR J=ll TO N:WEITE //2,X(J)/SCALE:NEXT J 
1910 PRINT //1,"$MA": INPUT //I, D$: PRINT D$: CLOSE #2 
1920 INPUT "MAKE ANOTHER RUN OR STOP R/S ",ANS$ 
1930 IF ANS$="R" THEN 19A0 ELSE 1960 

1940 INPUT "ENTER NEW VALUES OR USE CURRENT SETUP N/C";ANS$ 
1950 IF ANS$="N" THEN 1030 ELSE 1290 
1960 STOP: END 



APPENDIX B 
ELLIPSOMETRY 



When an elliptically polarized beam of light is reflected from a 
surface, its polarization direction is changed by the refractive index 
and absorption coefficient of the reflecting surface. If the surface 
is a thin film, then the film thickness also enters into the reflection 
equations (Drude Equations). 

Ellipsometer instrximents can be purchased with prepackaged 
computer programs to calculate the film thickness, refractive index and 
extinction coefficient from the ellipsometry data. 

We recently acquired a Gaertner Ellipsometer and conducted tests 
with the following results. 

a) While the ellipsometer uses a laser source, and 0.01 degree of 
resolution, for the polarizer and analyzer settings, the data was not 
accurate enough to measure a standard. 

b) The program supplied was adequate for substrates with vastly 
different indices from the deposited films, but was totally useless for 
systems where the indices were even relatively far apart, such as CdS 
films on silica substrates. 

Therefore, an intensive analysis of the sources of error in both 
the experimental apparatus and in the calculation program was 
conducted. The results showed that with some modifications in the 
instrument and with a new program, the ellipsometer can be used very 

230 



231 

accurately to measure the film refractive index and, if the thickness 
is approximately known, to measure the film thickness. 



Instrument Modifications 

Sources of error in an ellipsometer are numerous but only a few 
have a large effect on the results. 

1. Polarizer, Analyzer. Their accuracy is very adequate (0.01°), 
even for fine work. Their relative alignment can be checked by placing 
the arms of the ellipsometer in line with each other. After removing 
the compensator the positions of maximum transmission and extinction 
can be checked. Angular tracking can then be checked by rotating the 
polarizer. 

2. Quarter wave plate compensator. Our ellipsometer has a fixed 
compensator at +90°. By inserting it into the beam, its position can 
also be checked. If the compensator, is defective, it can still be 
used with a slight modification of the equations. 

3. Incidence Angle. This is the most critical adjustment in the 
instrument . A variation in the angle between source and detector of 
less than 1° can change the refractive index in the first place. We 
found that despite a well collimated laser source, our instrument was 
designed to accommodate changes in the angle of incidence of more than 
several degrees. This was done by using a wide angle collection system 
with a 2cm x 2cm photocell. This very sloppy alignment was used in 
order to collect light from samples which were not plane parallel. 



232 

Here a modification was necessary. We placed a small aperture at 
the entrance to the collection optics. Then we placed the detection 
photocell at the end of a 30cm long tube, behind a second small 
aperture. The two apertures, separated by 30cm reduced the acceptance 
angle of the detector system to: 0.2 degrees. With the arm of the 
detection, this was further lowered to 0.05 degrees. 

In order to accept non planar or parallel samples, the stage 

was adjusted to tilt until the reflected light was aligned 

with the detection system. 

4. Multiple reflections. Imperfect substrates and thin 
substrates can produce multiple reflection beams, arising from 
substrate striae or the bottom surface. These should be masked and 
only the top surface reflection should be received at the detector. 

Using the above alignment procedures, two standards were run every 
day and the angle was adjusted when necessary. Care was taken not to 
change the alignment until the standards were again measured. An 
accuracy of the Ath place in index could be obtained. 

Data Analysis 

Ellipsometry equations for the calculation of index and thickness, 
require a measurement of the quantities x and A, as shown below. 
However, the instrument yields polarizer Pi and analyzer Ai readings. 
These readings can be used to calculate the values of x and A, however, 
this operation is not straight forward. 



233 

The various polarizer and analyzer readings fall into four sets of 
readings called zones, two zones with the fast axis of the compensator 
set at +45° and two zones at -45°. There is one independent set of 
readings for each zone, giving four independent sets. However, both 
the polarizer and analyzer may be rotated by 180° without affecting the 
results, thus yielding 16 sets of readings, which can grow to 32 sets 
if the compensator is rotated by 180°. 

Depending upon which set of readings one is making, the 
calculation of x and be different. Furthermore, Brewster's angle 

(tanijjg = "^/no) is about 56.3° at a glass-air interface and A changes 
phase by 180° at this point. When working at ellipsometer angles of 50 
and 70°, certain film substrate configurations can develop which cross 
Brewster's angle. This leads to the necessity to change the x> ^ 
definitions. 

Calculations 

The equations are well known Drude Equations which will not be 
reproduced here. The reader is referred to Azzam and Bashara, 
Ellipsometry and Polarized Light , North Holland 1977 and reference 
number 6A. A two-step approach which was developed to solve the Drude 
equations to determine the refractive index and thickness will however, 
be outlined here. In the first step, ni is calculated using a Lotus 
routine for plotting, as follows: 

a) The data is used to calculate x and A. 



23A 

b) A wide range of ni values and d values is selected and the 
Drude Equations are solved for Xc ^c* 

c) A graphical comparison of the measured x> ^ is made with Xq ^^'^ 
Aj, for each set of n values over the range of d values. Since d varies 
cyclically, it is not important which d values are used as long as they 
span the cycle (A). 

d) This allows a rapid calculation of n since the approach of the 
calculation to the data can be seen in the plots. 

e) n can be calculated with desired accuracy. 

f) Once n is known, d can be calculated from the Drude Equations 
using the well known quadratic formula, with possible solutions at 9, 
-9, 2mir-9, 2mTr9 where m is the integer order number. Thus the 
thickness calculation cannot yield a unique value. However, if one 
knows the approximate thickness, so that m may be defined, then an 
accurate value can be obtained. 



BIBLIOGRAPHY 



1. T. Y. Chang, "Fast Self -Induced Refractive Index Changes in Optical 
Media: A Survey," Opt. Eng., 20, 220 (1981). 

2. M. Warren, Y.H. Lee, G.R. Olbright, B.P. McGinnis, H.M. Gibbs, N. 
Peyghambarian, T. Venkatesan, B. Wilkens, J. Smith, and A. Yariv, 
"Fabrication and Characterization of Arrays of GaAs All-Optical 
Logic Gates," Optical Bistability III , ed. by H.M. Gibbs, N. 
Peyghambarian, and H.S. Smith, (Springer-Verlag, Berlin, 1985), 
p. 39. 

3. M. Dagenais and W.F. Sharfin, "Fast All-Optical Switching at 
Extremely Low Switching Energy in CdS Platelet," Optical Bistability 
III , ed. by H.M. Gibbs, N. Peyghambarian, and H.S. Smith, (Springer- 
Verlag, Berlin, 1985), p. 122. 

4. M. Dagenais, "Low Power Optical Saturation of Bound Excitons With 
Giant Oscillator Strength," Appl. Phys. Lett., A3, 7A2 (1983). 

5. B.S. Wherret and N.A. Higgins, "Theory of Nonlinear Refraction Near 
the Band Edge of a Semiconductor," Proc. R. Soc. Lond. A, 379 , 67 
(1982). 

6. D.A.B. Miller, S.D. Smith, and B.S. Wherrett, "The Microscopic 
Mechanism of Third-Order Optical Nonlinearity in InSb," Opt. 
Comm., 35, 221 (1980). 

7. R.K. Jain and M.B. Klein, "Degenerate Four-Wave Mixing Near the Band 
Gap of Semiconductors," Appl. Phys. Lett., 35, A5A (1979). 

8. A. Yariv, Quantum Electronics , (John Wiley and Sons, New York, 
1975), chap. 8. 

9. E. Abraham, C.T Collins, and D. Smith, "The Optical Computer," Sci. 
Amer., 2A8, 85 (1983). 

10. D.A.B. Miller, "Refractive Fabry-Perot Bistability with Linear 
Absorption: Theory of Operation and Caviety Design," IEEE J. Quant. 
Elect., QE-17 . 306 (1918). 

11. M. Dagenais, W.F. Sharfin, "Picojoule, Subnanosecond, All-Optical 
Switching Using Bound Excitons in CdS," Appl. Phys. Lett., 46, 
230 (1985). 

12. G. Dieter, Mechanical Metallurgy sec.ed. , (McGraw-Hill, New 
York, 1961), p. 139. 



235 



236 



13. W. Albers, "Physical Chemsitry of Defects," Physics and Chemistry 
of II-VI Compounds , ed. M. Aven and J.S. Prener, (North-Holland 
Publishing Co. Amsterdam, 1967), p. 137. 

14. B. Segall and D.T.F. Marple, "Intrinsic Exciton Absorption," 
Physics and Chemistry of II-VI Compounds , ed. M. Aven and J.S. 
Prener, (North-Holland Publishing Co. Amsterdam, 1967), p. 319. 

15. B. Segall, "Band Structure," Physics and Chemistry of II-VI 
Compounds , ed. M. Aven and J.S. Prener, (North-Holland Publishing 
Co. Amsterdam, 1967), p. 3. 

16. F.A. Kroger, "Luminescence and Absorption of Solid Solutions in the 
Ternary System ZnS-CdS-MnS, " Physica, 7, 92 (1940). 

17. R.E. Halsted, "Radiative Recombination in the Band Edge Region," 
Physics and Chemistry of II-VI Compounds , ed. M. Aven and J.S. 
Prener, (North-Holland Publishing Co. Amsterdam, 1967), p. 385. 

18. R.J. Collins, "Mechanism and Defects Responsible for Edge Emmision 
in CdS," J. Appl. Phys., 30, 1135 (1959). 

19. D.G. Thomas and J.J. Hopfield, "Optical Properties of Bound 
Complexes in Cadium Sulfide," Phys. Rev., 128, 2135 (1962). 

20. C.H. Henry, R.A. Faulkner, and K. Nassau, "Donor-Acceptor Pair 
Lines in Cadmium Sulfide," Phys. Rev., 183, 798 (1969). 

21. C.H. Henry, K. Nassau, and J.W. Shiever, "Optical Studies of 
Shallow Acceptors in CdS and CdSe," Phys. Rev. B, 4, 2453 (1971). 

22. C.H. Henry and K. Nassau, "Lifetimes of Bound Excitons," Phys. Rev. 
1, 1628 (1970). 

23. D.G. Thomas and J.J. Hopfield, "Spin-Flip Raman Scattering in 
Cadmium Sulfide," Phys. Rev., 175, 1021 (1968). 

24. R.C. Leite, J.F. Scott, and T.C. Damen, "Multiple-Phonon Resonant 
Raman Scattering in CdS," Phys. Rev. Lett., 22, 780 (1969). 

25. R.C. Leite, J.F. Scott, and T.C. Damen, "Resonant Raman Effect in 
Semiconductors," Phys. Rev., 188, 1285 (1969). 

26. K. Bohnert, H. Kalt, and C. Klingshirn, "Intrinsic Absorptive 
Optical Bistability in CdS," Appl. Phys. Lett., 43, 1088 (1983). 

27. R.K. Cook and R.W. Christy, "Optical Properties of Polycrystalline 
CdS Films," J. Appl. Phys., 51, 668 (1980). 

28. L.I. Maissel and R. Gland, Handbook of Thin Film Technology . 
(McGraw-Hill, New York, 1970), chap. 1. 



237 



29. J.H. Wohlgemuth, D.E. Brodie, and P.C. Eastman, "A Comparison of 
the Optical Properties of Crystalline and Amorphous CdS," Can. J. 
Phys., D8, 581 (1975). 

30. M. Cardona and G. Harbeke, "Optical Properties and Band Structure 
of Wurtzite-Type Crystals and Rutile," Phys. Rev., 1J7, A1A67 
(1965). 

31. E. Khawaja and S.G. Tomlin, "The Optical Constants Of Thin 
Evaporated Films of Cadmium and Zinc Sulphides," J. Phys. D, 8, 
581 (1975). 

32. M.H. Christmann, K.A. Jones, and K.H. Olsen, "Formation of 
Hexagonal Flat Tops on the Surface of Heteroepitaxial (0001) CdS 
Films," J. Appl. Phys., 45, 4295 (9174). 

33. J. Humenberger, G. Linnert, and K. Lischka, "Hot-Wall Epitaxy of 
CdS Thin Films and Their Photoltimescence, " Thin Solid Films, 121, 
75 (1984). 

34. M. Gracia-Jimenz, G. Martinez, J.L. Martinez, E. Gomez, and A. 
Zehe, "Photoluminescence of Chemical Deposited CdS Films," J. 
Electrochem. Soc, 131, 2974 (1984). 

35. B.J. Feldman and J. A. Duisman, "Room-Temperature Photoluminescence 
In Sprayed-Pyrolyzed CdS," Appl. Phys. Lett., 37, 1092 (1980). 

36. I. Lagnado and M. Lichtensteiger , "RF-Sputtered Cadmium Sulfide 
Thin Crystals," J. Vac. Sci. Technol., 7, 318 (1970). 

37. J. A. Thornton, D.G. Cornog, W.W. Anderson, and J.E. Philips, Proc. 
of the 16th IEEE Photovoltaic Specialists Conf., 737 (1982). 

38. I. Matril, G. Gonzalez-Diaz, F. Sanchez -Quesada, and M. Rodigiez- 
Vidal, "Deposition Dependence of RF Sputtered CdS Films," Thin 
Solid Films, 90, 253 (1982). 

39. I. Matril, G. Gonzalez-Diaz, and F. Sanchez -Quesada, "Temperature 
and Bias Effects on the Electrical Properties of CdS Thin Films 
Prepared by RF Sputtering," Thin Solid Films, 114, 327 (1984). 

40. I. Matril, G. Gonzalez-Diaz, F. Sanchez -Quesada, and M. Rodigiez- 
Vidal, "Structural and Optical Properties of RF Sputtered CdS Thin 
Films," Thin Solid Films, 121, 85 (1984). 

41. I. Martil, G. Gonzalea-Diaz , and F. Sanchez -Quesada, "Heat 
Treatments of RF Sputtered Films for Solar Cell Applications," Sol. 
Eng. Mat., 12, 345 (1985). 

42. F.B. Michwletti and P. Mark, "Ambient-Sensitive Photoelectronic 
Behavior of CdS Sintered Layers," J. Appl. Phys., 39, 5274 (1968). 



238 

43. J. Warnock and D.D. Awschalom, "Quantum Size Effects in Simple 
Colored Glasses," Phys. Rev. B, 32, 5529 (1985). 

44. J. Warnock and D.D. Awschalom, "Picosecond Studies of Electron 
Confinement in Simple Colored Glasses," Appl. Phys. Lett., 48, 425 
(1986). 

45. N.F. Borrelli, D.W. Hall, H.J. Holland, and D.W. Smith, "Quantum 
Confinement Effects in Semiconductor Microcrystallites in Glass," 
to be published in J. Appl. Phys. 

46. US Guns, Cambell, CA. 

47. J. A. Thornton and A.S. Penfold, "Cylindrical Magnetron Sputtering," 
Thin Film Prosesses , ed. J.L. Vossen and W. Kern, (Academic Press, 
New York, 1978). 

48. J.L. Vossen and J.J. Cuomo, "Physical Methods of Film Deposition," 
Thin film Processes , ed. J.L. Vossen and W. Kern, (Academic Press, 
New York, 1978). p. 38 

49. E.M. Clausen Jr., "The Gamma to Alpha Transformation in Thin Film 
Alumina," Master's Thesis, University of Florida, (1985). 

50. P.C. Baker, "Trace Element Analysis of Limestone by Secondary 
Fluorescence X-ray Spectroscopy," Master's Thesis, University of 
Florida, (1981). 

51. J.I. Goldstein, ed. Scanning Electron Microscopy and X-ray 
Microanalysis , (Plemiin Press, New York, 1981). 

52. R. Swaneopeol, "Determining Refractive Index and Thickness of Thin 
Films From Wavelength Measurements Only," J. Opt. Soc. Amer. A, 2, 
1339 (1985). 

53. E. Hecht and A. Zajac, Optics , (Addison-Wesley Publishing. Co. 
Reading, MA, 1979), p. 306. 

54. T.S. Moss, Optical Properties of Semiconductors , (Butterworths 
Scientific Publishing. London, 1959), p. 34. 

55. J.E. Cline and S. Schwartz, "Determination of the Thickness of 
Alumintim on Silicon by X-ray Fluorescence," J. Electrochem. Soc, 
144, 605 (1967). 

56. L.I. Maissel and R. Gland, Handbook of Thin Film Technology , 
(McGraw-Hill, New York, 1970), chap. 15. 

57. A. Joshi, "Auger Electron Spectroscopy," Metals Handbook Volumn 10: 
Materials Characterization , 9th ed, (American Society for Metals, 
Cleveland, 1986), p. 549. 



239 



58. D. Briggs and M.P. Seah, Practical Surface Analysis by Auger and X- 
ray Photoelectron Spectroscopy , (John Wiley and Sons, New York, 
1983), p. 119. 

59. S.W. Gaarenstroom and N. Winograd, "Initial and Final State Effects 
in the ESCA Spectra of Cadmiiam and Silver Oxides," J. Chem. Phys., 
67, 3500 (1977). 

60. K. Urbanek, "Magnetron Sputtering of Si02; An Alternative to 
Chemical Vapor Deposition," Solid State Technol., April, 87 (1977). 

61. V.E. Mashchenko, "Interaction of Excitons and LO Phonons in CdS 
Under High Excitation and At High Temperature," Opt. Spectrosc, 
A2, 116 (1977). 

62. B.D. Cullity, Elements of X-ray Diffraction , (Addison-Wesley 
Publishing Co., Reading, MA, 1978), p. 458. 

63 B.D. Cullity, Elements of X-ray Diffraction , (Addison-Wesley Pub. 
Co., Inc., Reading, MA, 1978), chap. 11. 

64. E. Elizalde and F. Rueda, "On the Determination of the Optical 
Constants n(X) and a(A) of Thin Supported Films," Elect. Opt., 
122, 45 (1984). 



65. K.A. Simmons and J.H. Simmons "Graphical Method for Solution of 
Ellipsometry Data" to be published in J. Appl. Phys. 



BIOGRAPHICAL SKETCH 



Edward M. Clausen, Jr. was born December 11, 1958, in Chicago, 
Illinois. He graduated from Eastlake North High School in 1977 and 
began his college career at Cleveland State University, in Cleveland, 
Ohio. After transferring to the University of Florida, in Gainesville, 
Florida, he received his Bachelor of Science degree in Materials 
Science and Engineering in August of 1982. 

Edward began his graduate studies at the University of Florida in 
January of 1983, and received a Master's degree in Materials Science 
and Engineering in May of 1985. He is currently working to complete 
the requirements of his doctoral degree in Materials Science and 
Engineering. 



2A0 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 




Joseph H. Simmons, Chairman 
Professor of Materials 
Science and Engineering 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 

Paul H. Holloway, 
Professor of Materials 
Science and Engineering 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 




Robert T. Dehoff, 
Professor of Materials 
Science and Engineering 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 



I certify that I have read this study and that in my opinion it 
conforms to acceptable standards of scholarly presentation and is fully 
adequate, in scope and quality, as a dissertation for the degree of 
Doctor of Philosophy. 



This dissertation was submitted to the Graduate Faculty of the 
College of Engineering and to the Graduate School and was accepted as 
partial fulfillment of the requirements for the degree of Doctor of 
Philosophy. 

December, 1987 




Stanley R. Bates, 
Associate Engineer of 

Materials Science and 

Engineering 




Timothy J. Anderson, 
Professor of Chemical 
Engineering 




Dean, College of Engineering 



Dean, Graduate School